I guess I'll be first to say something....again....
For new players, starting out as a mage or knight as your first character is a bit difficult since mages lack HP and def; and may take a while to level up. Knights lack dmg but upside is higher HP and def; but low attack power usually means grinding/questing would take longer well....assumming you are doing it alone. Decent enchanted gear would solve the problem easily but, since new players don't really have the needed funds for that immediately like mentioned earlier gameplay may be a bit difficult.
Warriors and Explorers would be alright to use at the start. Warriors have higher damage output and a decent amount of HP, but lacking the ability to use a shield. Though, if you dont charge into a massive mob then you'll be fine. Explorers can use like 4 different types of weapons; daggers, knuckles, crossbows, bows. (Warriors can use spears, two handed swords, knuckles. Knights uses maces, one hand swords, knuckles. Mages uses staves, daggers, and knuckles. All beginner classes are able to use knuckles but, only guardians [one of knights advance classes] can specialize in them.) Explorers somewhat lack HP, and damage output is a bit lower; but because of their higher luck they may deal critical damage often. Regardless of decently enchanted equipment warriors and explorer should do alright up til 80.
As for leveling, do the first quests in belos. and gradually grind from forest area, from attacking prirings/beetles up to LV5, wild dogs/wolves up til Lv6 or 7. Then on to ancient forest area grind off of silver wolves or boars up to around Lv10. Move up on to the temple of pluton (portal is located above the one from Elias portal) grind off of bogles (in second room) up til lv15 ,they may be a bit difficult since they start to damage you more and can be a bit dangerous to mob. After Lv15 or 16 head up to the highest room of temple of pluton (use the world map for reference) and grind off of goblins up til 20.
At 20 up, you may get the digging skill from a quest from Ases in the far right side of belos, and gather quests from the fight arena, and/or elias palace for behemoth stomach quests, and dig around the field area right of elias and grind/quests there up til 24~25; or you can go past yong gyoung (third town of the game, use map for reference) and level off of the black beetles until 26~27, then level off of the pumpkins in the next field over til Lv.30.
From 30 up, most people usually party up and grind in spooky village from 30~45, 58~80, 88~110. Exp is decent over there and you should level faster. Having exp boosting items would helpout (i.e. Advance XP potions, 1~80 XP nostrums) or you could continue from going off to arcadia grind and quest there, then later progress to cherry lake, dark moon castle in aoich, desert, pyramid, ocra beach etc. etc.
For quest references, view Pie's Quest Guide; located in the LaTale Guides -> LaTale quest guides of the forums here. ( I would have made a link here but, I forgot how to do that :|)
If you have any other questions you could either pm me, or other people in here, or simply ask around from in-game.