They're all pretty simple to learn. Bring up the skill window with X and put points into the top row (5 points unlocks the next skill in line) to try out the class-specific skills.
Every class starts off really slow, mostly because you start with no armor and everything from squirrels to beetles kicks your butt. The exception is water mage, because it can heal itself with AOE water spells from the start.
Ranger (any spec) and Water Mages are straightforward, and are easily the most overpowered classes. Between either, it's a matter of whenever you want a fast way of moving around with an exploit or want a heal spell for the sake of never crafting food/potions ever again.
Warrior is initially underwhelming, but Guardians can become defensive juggernauts later on if you don't want to rely on exploits and heal spells.
Rogue isn't so different from Warrior, with only different looking weapons (Greatswords/Battleaxes/etc are Longswords and swords/maces/etc are fists and daggers) and a stealth skill in place of a spin attack. Attack, fill up your MP bar, special attack, repeat. Ninja is the way to go.
(I know this is pretty late to the table, but I figured I'd post for anyone else asking the same question.)
I have to disagree with the last two posts I see here.
Picking a ranged class (Ranger/Mage) is a pretty poor choice if you want to learn the mechanics of the game. You never really learn the attack patterns of enemies and you don't really get a chance to learn when you should or shouldn't roll/dodge.
Learn how to play a warrior first. I suggest playing as a guardian, and use your blocking ability to counter/parry/learn their attacks and their frequency. After you know that, and when to roll/dodge, everything falls into place, regardless of what character you play as.