On the Xbox it's the LT on your Controller.
In my Manual (Xbox) I read about the LT :
ModeIf you want to concentrate on a single enemy, you can lock onto it by pressing and holding LT . This is especially handy if you're attacking with bows and/or long range spells. During lock-mode you can switch targets by nudging the R-stick sideways.
For the PC, sory, I don't know.