http://iro.ragnarokonline.com/When you start out in this game you start as a Novice, and the first 15 days are free. (Not in real life. In game time.)
When you level up to a certain point you can advance to the next job. You can pick from...
Swordman, Mage, Archer, Merchant, Theif, and Acolyte. Each of those classes are different, but if you want to take a different path wait for a surprize.