At its most basic level, there are 3 types of classes. Healers(3 flavors), tanks(3 flavors), and DPS(7 flavors). There are differences in the flavors(bard doesnt hit as hard as monk but offers more party utility, scholar blocks more damage from happening while white mage heals harder), but for almost all content in the game they are basically interchangeable. The person playing paying attention to whats going on and understanding their class means a lot more then the differences between the versions in all the "reasonable" content. So if you have a type that you like to play then just go for it and you'll be fine. Alternatively, play 1 of each(tank, healer, DPS) up to level 15. At 15 a series of training missions open up(hall of the novice) to give you a feel for how group situations may go with each role(so again: 1 for healer, 1 for tank and 1 for DPS). You can try those out and see if something feels right. Or you can ask more questions and someone here might be able to point you in the right direction. Each class does play a little differently at end game(even though both are melee-based DPS classes a ninja and a dragoon dont really feel all that similar), though they all start out pretty close to the same and gradually branch out as they get bigger.

Hope some of that helps. Keep asking questions and trying new things and youre sure to find your way.