Answer:
Depends on the guy, if he seems sweet and cares about who you are, instead of your boobs, then generally he'll want to know that he can care for you. Don't just walk up to him and say "You. Boyfriend. Now." Start as a friend and try to learn about him, what you have in common, stuff like that.
As for the popular guy who cares for only boobs, give him what he wants. (Feel free to improve)