What do I do if I like this guy but I think he likes a different girl? HELP! What do I do?

If he is a relationship, then you leave him alone.

If both of you are not currently in relationships, then ask him out on a date. You don't have to declare your feelings for him to ask to a movie. If he says yes, then have fun. If he says no, then you haven't lost anything. You've actually gained some self-confidence either way because now you know you can face a fear and ask a guy out instead of waiting for him to do the asking.

I definitely agree with Cat. I think that if both of you are single, ask him if he wants to hang out. Maybe in a group first so it's not awkward. If he comes with, talk to him, and maybe he'll change his mind about this other girl and want to go out with you!