Welcome to the mana world!
First off you don't select a "class" type like in other games. Here the way you set up your stats determines if you could be best suited for one or another class.
Let's talk about the pro's and cons of each class.
Warrior, uses swords, uses heavy armor(warlord plates and such)
Pro: Possibly the most versatile class out there, everyone begins as a warrior, and not much stat tweaks are needed.
Con: at times a few enemies may be too much for you, extra potions and other items will be needed. You most likely will have to work in groups to do a few quests at higher up.
special:*usual suited for attacking one large difficult enemies. Works well in groups of other warriors/mages/archers
Archer, uses bows, can use heavy armor with one difference from a warrior
Pro: "distance is my only armor" hitting from far away can keep your health high once you master this skill. With a few tweaks on stats you can hit hard, and fast depending on the type of archer you want to become.
Con: arrows.... a cost that can be dealt with but you'll have to stop a few times to make a few runs to get money, arrows and supplies. Another disadvantage; you can't use shields so you are less protected than a warrior(however if you can keep a distance you'll be fine).
special: *usually suited for attacking mobs of enemies with some training, great as a back up for a group, but can work better alone picking off enemies.
Mage, uses MP, and inner magic(or items) to cast spells of different classes. (offensive and defensive, stat changing, healing)
Pro: A fairly new class added to the mana world (2009) that is now used to wreck havoc on some tough enemies. Deals the most deadliest attacks, healing, minor summoning, and stat enhancements (such as protection spells).
Con: I personally recommend this AFTER you lvl up a bit with warrior or archer, the mage has weakest defense, and it takes time to be a mage, along with other quests to do find trainers/teachers who can properly train you in different magic classes. At the early stages you'll need to horde common/special items that would be used for magic.
Special: *a well trained mage is a deadly force to be reconned with, many warriors befriends one due to their healing ability, at later stages mages can attack on their own, and alleviate the item hording problem. But much more suited to go in groups of protecting warriors, archers.
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Ending notes:
I always say this, just play your style.

Do what you'd like to do best and work with it. the best part about this game is that you're not STUCK to one class. by changing your stats you can go back to warrior or archer as you choose.