Well, it's different from guy to guy. As for me, the first thing that hooks me is a girl's smile and eyes...I've said before that I'm a damned sucker for a girl with gorgeous eyes and a cute smile. But the main driving point is the girl's personality. She has to be fun to be with, lively, ya know? But, like I said, this is just me. Some guys like different things.
Note to you: prepare for some guys to say things like "I like t*ts and ass" and other crap like that. Just a warning.