I can't find where to assign spells and abilities to the buttons on the controller (I'm playing on the 360).

How can I assign new abilities to the A and B buttons, and any other buttons that are available? While I'm at it, I can't figure out how to switch between ranged and melee weapons either? I'm incredibly hopeless at this game right now.

Sorry to be such a fantastic noob, but I would be grateful for any help.