How to ask him out?

Well since you have never really talked to him before, if you guys have a class together ask him something about that class. And whenever you see him the the hall, say hey to him. Just acknowledge him for awhile until you're comfortable like asking him to hang out sometime like going to the movies with a group of your friends and his friends combined, so you can get to know his friends. Just get to know each other first, and then ask him to prom and if that goes well, ask him out.

Hope everything works out for you, and you get over your shyness. Just try to come out of your shell a little. :)