Classes and Skills are more liquid in this game than in Diablo II. The Ascendancy class is the most important choice, I hope it will give you a fun playing even if your character so random skills that make little thematic sense.

e.g. I'm playing a Duelist, but all I do is hit my enemy with sword/axe, seeing 4 Balls of Molten Magma hitting anything around it (but me).