How to build your character is one of the most often asked questions.
First off, realize Mabi has no classes. It has destinies now as of G13, but you are free to rank any skill you want, use any style of combat you want, and switch at any time you please. If you stick with Mabi long enough, you can and will eventually completely finish out every skill possible for a given style of combat, and ultimately every single skill in the game. Every. Single. One. of. Them. Now, realistically, unless you are the most dedicated and hardcore player, willing to spend a fairly large amount of real life cash on NX to speed the process up, it will probably take a couple of years to reach that point (and it will still take a long time even with NX!).
Being able to accomplish goals like gaining levels, beating dungeons/missions, and acquiring wealth and stuff requires some power, some ability to get opponents dead. You don't need every skill in the game maxed out to do this however, nor do you even need a lot of skills. What you really need are one or two powerful main attacks, and a bit of focus on those attacks. With those you can function fairly well in the vast majority of situations you may find yourself in Mabi. Thus the concept of "builds." A build is not the same as a class in another game. You're not restricted ever. You can start a build when you are brand new to the game, or after you have already been playing awhile and ranked numerous skills. You can abandon a build at whim. But if you focus how you spend your AP in certain ways, then you will find yourself growing more powerful than if you just spread yourself out across multiple areas.
A build is a sort of guide for yourself on how to spend your AP, training time, and to some extent the sort of equipment that works well with the build.
A build won't make you able to win everything in Mabi. You won't be soloing Peaca with the results of this guide but you should find yourself with an easier time tackling the sorts of missions and dungeons people will suggest you be spending time in to do your leveling or to gain "easy" money. By having an easier time with those basic aspects of gameplay, you can more easily gain AP needed to do well in those niche places like Peaca, buy more expensive stuff with your new found wealth earning potential, and so on.
Here are some loose guidelines when following a build:
The more you focus, the less AP you spend on things that don't help you -now-. By doing so you have more AP to spend on those things that do help you now, and more quickly gain power. That said, it is permissible to get any skill you want to rank F. The AP costs are low, and the best way to try a skill out is to get it and use it. Any drawbacks there might be for ranking a skill to F are minimal at best. In fact I highly encourage you to get any skill you can learn unless you know very specifically what you want to do. Even then, rank F skill can be used to good effect by a skilled player, and will give you flexibility. It doesn't matter if the skill is outside your build or not, if you can learn it easily then do so.
Do not however go around ranking tons and tons of skills beyond rank F. Just because a skill has a shiny yellow "advance" button next to it and you happen to have 20 or 30 AP, don't do it.
The Guide
Step 1: No matter what your build may be, accumulate power. While rank F skills and personal competence will let you beat an opponent, rank 1 skills are easier and more powerful to do the job. Pick one skill. Get it to rank 1. It doesn't matter what that skill is, but focus on that. Use that powerful skill to make getting levels and money easier for you. That said, some skills are more effective than others; I'll go more into that detail later. But for now, if you're having a hard time picking, choose a skill that is spammable. Meaning you can load it quickly, do damage, and keep one thing (at least) from killing you. Also, ignore defense. Offense gets things dead and dead things can't hurt you. Trying to make your character a tank takes a lot of effort, a lot of equipment. Starting out by improving your offense will get you the strength and means to more easily acquire the equipment and skills needed to make your character more survivable. For now, learn now to use your attack either by itself or in combination with other skills to never get hit. Use basic, rank F skills like Defense, Counter Attack, and so on to accomplish this if you need to.
Step 2: Now that you have the ability to use one skill to do decent raw damage, it is time to flesh your character out a bit more. If there are skills that compliment your main one, work on those. Try to keep your focus in a narrow field of combat and the narrower the better. By compliment, I mean that by ranking these complimentary skills you can continue to use your main attack, and these skills will make it stronger. Critical hit falls under this category, you want this skill rank 1, no exceptions. Other probable candidates for this step are things like the various mastery skills that came out with g13, or other passive skills, or things that add a good amount of an important stat (like STR, DEX, INT, or MP/SP).
Step 3: Make your area of combat apply to more situations. If your spammable skill can only hit a single target, then you may want to invest in some sort of skill that hits an area. If your main skill is somewhat slow to use, you may want some faster skills. If your main skill is fast but low damage you may want a slow but powerful skill that can do high damage. If you have ranked some mastery skills or other passive boosts or worked on a particular stat, then choose skills that will benefit from the work you have already done. I.E. if you chose a melee skill for the first step, and you ranked combat mastery, critical hit, and sword mastery for the second step, pick a melee skill for this step that will benefit from all that work. Windmill and smash are great examples for this step.
After completing this third step, you should be powerful enough to kill most of your opponents. No one should reasonably expect you to solo Peaca or anything like that yet, but you should be able to grind out exp and money for yourself or handle your own in a party.
Step 4: Use this power you have accumulated to build up some equipment you will want. You will want some gear that is useful for your main build, and you'll want to seriously collect some -CP gear.
Everyone needs -CP gear, no exceptions. If you have good friends and/or a helpful guild that can loan you some, great, but if you have your own then it is available at all times, and you don't need to worry about someone else using it when you need to do some training. -CP gear makes nearly all skills easier to train than not having it. -CP gear is not something you use to fix the problem of having high CP, it is something you use to make your training easier, regardless how high or low your CP is.
Your gear that helps your build does so by boosting your damage. Higher damage equals faster killing, which equals faster exp/money.
Do note that if you are especially proactive, you can work on step 4 from the start, but step 1 shouldn't take too long, and is more important. By all means keep an eye out for good deals when you see them though!
Step5: Congratulations, you've formed a basis of a powerful character. You can at this point continue to specialize in a single area. Continue to repeat steps 2 and 3, choosing skills and boosting effects that don't necessarily affect your main attack directly. Repeat step 4 by accumulating items or life skills that make your life easier. Potion crafting for example is helpful to lots of characters to have a supply of potions on hand more readily.
Alternatively, you can branch out into other fields. Specilization leads to greater power by building upon what you're already strong at, but diversification will open up more possible ways to play. Both options have their strong points. Should you choose to branch out instead of specialize, use your existing powerful style to grind AP you need, then use your -CP gear to easily and quickly train your new skills til they are strong enough to suit your needs.
Examples
Here are some of the most popular/effective builds. By all means feel free to come up with your own, use the ideas I post below in part or in full, or ignore them completely. Your primary concern should be with picking something that will be fun for you. These suggestions are based around the idea that it is fun to be self-sufficient to some degree, and capitalize on some of the more powerful skills/sets of skills in the game.
Windmill (melee): This build focuses on the skill Windmill for step 1. This skill is especially popular for humans and giants, though less popular for elves due to the harder training and less powerful results. Windmill is a fast skill (1.6 second load time to start, shortens to 0.8 seconds at rank 9), hits an area with unlimited targets, does good damage, knocks things away from you, and has several defensive as well as offensive uses. Being fast, good damage, and area effect make this arguably the strongest build to start with. As this skill is a melee skill, you should obviously get into the warrior destiny as soon as you can.
Your second step should focus on critical hit, combat mastery, sword/blunt mastery (depending on race, swords for humans, blunts for elves and giants). All of these will boost the damage of your already powerful windmill skill and have you clearing out fomors at an astonishing rate.
Your third step should focus on melee skills. Smash is a great compliment to windmill. Assault slash (new g13 skill) is also incredibly good. You can also grab skills that boost STR like final hit (human) taunt/throw attack/wind guard/stomp (giant), charge, and carpentry.
Your fourth step should involve getting enchants to boost your damage. As long as your critical rate is 30% you should be fine for most situations since windmill ignores protection. Windmill itself, critical hit, and smash all raise will, so between your weapons, title, and gear you should easily be able to get the remainder you need, if any, to maintain that 30%. A stiff tailcap can easily solve all your needs, granting 10% and is really cheap. Yet another reason windmill is such a good choice to start with, you can focus nearly all your effort into boosting your damage.
On the other hand, windmill is also one of the more tedious combat skills to train in the game. If you can get your hands on some -CP gear early, it will help your training quite a bit. Barring that, you will want to really focus on WM itself and limit how much of step 2 you try to do early in order to keep your CP low.
For step 5, if you seek to take this build all the way to the end, you're going to start dipping into a lot of skills that aren't particularly useful to you, or even all that good in the pursuit of more STR. Skills like arrow revolver, fireball, and blaze aren't even melee skills, but add to STR. You're going to want to boost your DEX to improve your balance as the best weapons that do the most damage have low balance. Archery and DEX life skills are good for this. In the end, this build very closely resembles the next one.
Windmill(archer): Much of this build starts out the same as the melee build. In fact you can follow the melee build as far as you feel is helpful to you for step 1 and 2, in order to be able to level decently well. However your goal is to use WM as a platform of earning AP in order to more easily rank up the skills of your primary method of fighting: ranged combat. You'll probably want to start off in warrior destiny to train windmill and your support skills for it easier, as well as to boost STR. Once you switch over to training range skills you'll have to weigh the benefit of easier training for those skills via archer destiny or remaining warrior for stronger windmilling. As giants don't have range skills of note (throw attack is STR based) this build is mostly for humans, though some elves may choose it as well.
The strategy of this build is that range skill itself is rather poor for a human, and is mostly a passive boost for other skills like arrow revolver or magnum shot... but those skills are challenging to use well by themselves (arrow revolver even requires a quest to get that can be out of the capabilities of a newbie to complete). Elven range doesn't really suffer this problem, so most of this build will assume a human. If you're an elf following it, adjust to your needs.
Your eventual goal with this build will end up turning windmill into a defensive skill and a source of Will stat for crits. But you can use it at the start to fuel your AP needs. Your first diversion from the melee build above will be in step 2. Use whatever melee passives you want to boost your windmill, as combat mastery and sword mastery both eventually add to DEX which is useful for range too. Critical is obviously good. But before you rank any of those to where they do actually boost your DEX, rank the life skill refine to at least rank 2, and get as many mithril fails done as you can stomach (ideally all of them).
Once you have achieved this checkpoint, you can finally start working on what will become your main combat style. Build ranged attack itself finally, though you will probably split your time between training range, and doing your real fighting with windmill for awhile. Use windmill to help you get arrow revolver quest done. A good checkpoint to work towards at this stage is rank1 ranged attack and rank5 arrow revolver. At some point along the way of working toward this goal, you should find yourself able to use range to good effect, and it may situationally be more powerful than your windmill, but don't be afraid to rely heavily on melee, as windmill is a really powerful skill.
Your step 3 will be a really long, tedious process of ranking up arrow revolver the rest of the way to 1, magnum shot to 1, crash shot to 1, support shot to 1, weaving to rank 1, production mastery to 1 and grabbing anything else that adds even a little bit of DEX. Feel free to take a break from this and work on some melee if you like. I suggest starting weaving with 180+ DEX and rank 1 production mastery already in place.
Your step four is only slightly different than melee's. Since your main method of combat will start out focusing on arrow revolver for quick spammy damage but ultimately be magnum shots, crash shots when applicable, and arrow revolver to clean up strays. Arrow revolver and magnum don't ignore protection for their crit rate, and crash shot requires a decent sized spawn of mobs to be effective. So your crit rate will be very important when choosing things like enchants for your gear. However since you will have access to crash shot and windmill, you can also focus on max damage. To really shine you are going to want basically two sets of gear: one focused heavily on crit, one focused heavily on max damage. You can then interchange pieces as needed to get enough crit for what you are fighting, and keep your damage as high as possible. For flexibility reasons, clothing is highly recommended because accessories are very small when sitting in inventory, and are great for swapping out to fit different situations. Since melee/windmill is fairly strong in this build enchants that boost DEX or STR are not quite as good as enchants that directly boost max damage. An enchant like Wild Boar (up to +3 max damage) may be more valuable to you than a DEX enchant that gives +4 damage worth of DEX, since Wild Boar will apply wither you are using your bow or swords.
Range (pure): This build is most popular with elves, as their range skills are pretty effective early on, and training a skill like windmill is harder than it is for humans. Archer destiny is an obvious choice. Again, giants can't be effective rangers. Your step one will be focused on ranking refine and ranged attack. Rank ranged attack enough to where you can kill things more easily, but not so high that you will have a hard time getting mithril fails at the end of refine training. Even more than for a human, you will want to invest in some -DEX gear like scarecrow to make refine training as easy as possible and get it done and over with. Your starting bow should probably be either a guardian bow or an llb. Bows in general aren't terribly expensive to repair, but bows other than guardian bow supposedly lose dura for each arrow an elf fires, while the guardian bow loses dura per attack (in other words, half as fast for an elf). Leather longbow offers better range and stun options though, and will perform better in combat for your newbie elf. Since bows gain proficiency really fast (especially now in g13) upgrades will become a factor very quickly. I recommend just going with anything that boosts your range or improves damage without sacrificing range for your starting bow. An Elven longbow may also be a good choice for the excellent range it gets from upgrades.
Your step 2 will involve grabbing anything that boosts DEX. Put weaving off til you have around 180 DEX and have completed production mastery. Don't touch production mastery til you have refine past the mithril fails part. Some of this will mingle with your step 3, which will be to rank magnum as fast as possible. Elven magnum is incredibly powerful, and having both range and mag at rank 1 will make all the hard efforts you've had up to this point worth it. Welcome to having the best single-target DPS in the game. Now work on boosting it even better with more DEX stuff. Crash shot is part of step 3, but also adds incredible power to this build via finally having an area effect attack.
For step four, in addition to the -CP gear all builds should get, you're going to really want to focus on getting a critical boosting set. This is of much higher priority than a damage set, unlike previous builds. Your primary attacks are magnum and ranged attack and you want access to as much crit as you can get your hands on. Like the WM-range build above though, you will want some damage boosting options, but you can freely grab whatever gives you the most bang for your buck be it max damage or DEX boosting. You'll want to work on getting a high-crit bow like a composite bow or eventually a wing bow (expensive). When working on higher damage options for your collection of gear, you'll want a high max damage bow like a longbow or ring bow to use with crash shot.
Thunder (mage): This build is attractive to humans and elves alike, though even giants can follow it (they'll just spend more MP casting spells than the other two races). The allure of thunder is that despite it's seemingly high mana cost, this spell ends up dishing out so much damage and so easily hits multiple targets, that it ends up being one of the fastest ways to kill things for the least amount of mana among spells, proving to be far more efficient than any basic bolt spell. Even more so than human range build above, thunder is a challenging build to work with at the start. So you're going to Use another skill as a stepping stone like windmill is used above. I recommend a basic bolt like icebolt, as that will add to your int and eventually help your thunder out in the long run. You could use windmill or elven range as well, but your step 1 is to get something going that will make it easier for you to get AP because thunder needs a lot of it. Icebolt is a strong suggestion in this case as it costs very little AP itself and is probably one of the fastest routes to newbie power possible in the game. The early icebolt stages of your build will probably be better spent in alchemy destiny for the greater MP gains while leveling. Icebolt is ridiculously easy to train, so the double speed training of wizard destiny would be useless. Once you start training Thunder itself you may stick with alchemist for the MP or you might go wizard for the double speed training thunder; you'll have to weigh the pros and cons yourself.
Part of the long road of ranking thunder to 1 involves casting it a lot. This consumes a fairly large amount of MP. Fortunately thunder boosts that stat as you rank it, but you'll probably want more. Magic mastery and meditate will help with this a lot as being efficient sources of the MP stat while meditate helps in refilling your pool to cast more. Lightning Mastery will also help by boosting damage and lowering mp cost of your thunders. You're also going to want to invest in a lightning wand, take care of it, and upgrade it for -mp cost (and artisan upgrade as the 5th upgrade... chaincasting is bad, don't bother with it especially on your first lighting wand. Play with that stuff later on when you have the extra income to do so).
You may find thunder's damage erratic due to the low min/high max damage of the spell. Don't worry too much about that, and just focus on getting the skill ranked up. More ranks is higher damage, even if erratic. You can worry about boosting your INT to stabilize your damage more after you hit rank 1. I'll note that this advice is different than Trigger's popular guide for mages where he advocates building INT through basic bolt spells and things first, then moving on to thunder.
Since acquiring the AP for thunder alone is going to take you a bit longer than other build's main skills, you can spend some of the time you're training thunder by casting it in dungeons and things that accomplish step 4. Enchants that boost your INT and MP pool are especially useful, as is getting an mp cost reduction set of clothing. Will o' the Wisp and Sprout are excellent choices, and will last you through most of your mage career. CP gear won't likely be too important for training thunder itself, but by all means grab the easy pieces as soon as you are able to handle the dungeons they are found in. Thunder raises CP higher than most other skills in the game, so -CP gear will be even more valuable after you finish ranking thunder and want to move on to other skills.
One of the huge advantages of the thunder build, is that your step 5 already partially completes itself around rank 6 of training thunder. You can very effectively raft using thunder at this rank, and it only gets easier with higher ranks. Rafting effectively lets you (solo) easily get enough stars to get mana herbs, which a potion crafter can turn into mp100 potions. You use less mana than what you produce, which makes thunder start paying for itself in mana costs. Effective rafting is also the best source of exploration exp, which translates to an easier time getting AP, which in the long run makes thunder pay for it's own high AP cost.
Golem (alchemist): This build is open to any race, with no real advantage or disadvantage to any. Golem skill is a very powerful skill, though some say it is over powered and clamor for a nerf. Alchemist destiny will help immensely for some of the demanding training requirements of this build, I wouldn't even consider any other destiny at all. One major difference between this build and others is that your primary skill has some elements of a life skill to it in terms of training. Most of your training time is going to be spent gathering materials for the production of golem crystals. So while you will gain the most power from ranking golem itself, you'll probably find that a mix of ranking golem summoning, alchemy mastery, and perhaps even transmutation skill (for extra success rate in crystal making) will help with the production side of training the skill. I would also highly recommend purchasing and training an herb pet to decent levels as a source of white herbs. Your alternative is to try and spend some time earning money to buy white herbs from other players.
For your step 2, only two skills directly apply: alchemy mastery and clay mastery. All the golem crystals you make will be partially useful for training the former, while summoning golems will provide some training for both skills (at least the early ranks for clay mastery) but you're going to need to dip into other skills to train them both very high. In particular, AM requires several thousand kills be done on specific levels of opponents using alchemy attacks, for which golem doesn't count; some ranks in water cannon will be helpful for this grind. Clay mastery will start requiring you use an earth alchemy attack to train it, for which only sand burst applies. You will also want at least low ranks in synthesis and fragmentation for alchemy mastery, and unless you're fine with leaving it at rk2 you're going to have to devote a lot of ranks into mana crystallization (transmutation skill helps offset this some via making higher quality crystals) due to needing to make 1200+ (600 with alchemist destiny) crystals that are rank 2, 3, or 4 in order to get to rank 1 alchemy mastery. As far as an efficient build goes however, you should be fine with rank 1 golem summoning, rank 2 alchemy mastery, and rank 1 clay mastery.
Your step 3 has one pretty obvious candidate in the form of barrier spikes. It is a skill boosted by earth mastery and alchemy mastery, and will be stronger with the clay cylinders you are specializing in. It further is useful in protecting you. Less directly interlinked but still useful would be water cannon, which would give you an attack with which to train alchemy mastery, and provide you with a from of offense for situations where your golem isn't the best idea, and unlike flame burst can be fired over your barrier spikes. Life drain is an all around powerful attack and worth training as well. It is interesting to note, that unlike the other builds, critical hit doesn't boost your primary attack, summon golem. However it is still a great skill for boosting things like your water cannon attacks.
Step four is very easy for you. Since most of your combat involves your golem, the only gear that is absolutely needed for you is a clay or earthquake cylinder. You might consider a water cylinder for your water cannon attacks, should you choose to focus on that skill much, or a normal cylinder. You're free to run around nude/robed if you like, for very low maintenance costs (just the cost of your crystals and repairs on cylinder) or wearing whatever outfit you think looks coolest. If you do end up wanting to invest in some functional equipment, you can start working on a set of gear for another build: either alchemic damage enchants for enhancing water cannon/other attacks or you could start building a damage/crit set for melee or range, or you could work on a tank setup to protect you better for those times where you get attacked while controlling your golem. As golem summoning adds no CP, and alchemy and clay masteries only add 130 CP (though 48.5 stat CP between the two) your biggest increase of CP is likely to come from ranking water cannon. Still, some -CP gear may be of use for training alchemy mastery, and -CP gear will eventually be useful down the road... this build has very little dependency on it.
Other builds: As I said in the beginning, you can base a build around just about any attack. I have listed the ones I think are most effective in terms of how useful the skill is in general situations, how easy it is to acquire/train the skill as a newbie just starting the game, and how well you can mesh other skills with it to gain a high amount of power and/or effectiveness in a large enough variety of situations. Some noteworthy options that I am choosing not to create full entries for are:
Water cannon/Flame burst/Heat buster, aka the traditional alchemist that most people seem to suggest; These skills are close-short range skills that hit either a single target, or a very small area of effect. In order to make these skills do respectable damage, the player needs quite a few things going for them aside from merely ranking the skill to rank 1. Heat buster in particular is completely awful by itself and is really a support skill for one of the other two. But to create the "traditional alchemist build" and complete step 3 you'd need rank 1 flame burst, water cannon, heat buster, chain cylinder, alchemy mastery, water mastery, flame mastery, and either achieve royal alchemist position or complete a cressidia set for flame burst and/or water cannon set effects, and acquire all the various enchants. And even after all that effort, golem is still stronger due to having an enormous area of effect and very potent damage. Since our goal is to get strong fairly quickly, this build just seems plain inferior to start with. It would be a great style of play to work on once you have your basis in earth alchemy.
Ice spear, fireball, hailstorm; while thunder also requires a quest that involves hunting down pages, ice spear and fireball both involve getting twice as many pages as thunder requires, and involve one or more pages that are valued at roughly 1m on the market compared to thunders top page costing at most 250k, despite being -easily- hunted for in Rabbie/normal. A newbie mage with nothing but rk9 icebolt could farm the pages for thunder easily. Hunting fb10, is2, or ej3 not so much. If you want to create a build around these skills, you'll need to come up with some way to acquire these rather hard/expensive pages. As such they are out of the reach of your standard newbie. The point of these builds is a way to get the kind of power you need to get these pages. Hailstorm has as one of it's requirements that you already know Ice spear.
Blaze is certainly an incredibly powerful spell. However in order to make it do good damage, you need a rather hefty amount of INT. While acquiring the skill is really easy, getting the skill to the point of being highly powerful involves quite a lot more support-type ranking than thunder does. You'll need a decent amount of stamina and a really high mp pool to be able to sustainably use the skill, as well as a solid supply of mp pots. Since thunder is arguably one of the best sources of mana herbs (which in results in mana potions), it would be kind of hard to consider blaze a build when it involves ranking thunder to support it. Like the alchemy skill set, blaze is much better suited to an established mage rather than as a route to becoming one.
Bolt spells/fusion magic: basic bolt spells, much like the alchemy skills descibed above are either single target or limited area of effect. They lack the utility provided by a large area of effect like golem, windmill, or thunder, and they lack the raw power.
If you feel that any skill I haven't posted a mini-guide about would be an awesome foundation for a build, by all means work it, run it, and most of all have fun with it. If you take nothing else from my guide though, consider the basic concepts I laid out in the beginning. The important part is to focus at the start so that you can more easily kill things and convert them into AP/wealth and make everything else in your Mabi life that much easier.
Comments, feedback, pointing out of spelling or grammar mistakes welcome. I'll edit and fix as needed.