Do you want to lead the party and soak enemy damage as a tank?
Picking a class, however, is a much more intimidating problem. World of Warcraft has 12 classes, each having between two and four specializations (called specs) that determine what abilities they have and what role they play in a group. Do you want to lead the party and soak enemy damage as a tank? Want to keep players alive as a healer? Or do you want to pull off insane damage as a damage-dealer (DPS)?