Help liking a guy and not knowing what to do

I don’t know how to say I like this guy. I like him a lot but he has a girlfriend already, and I was the first person he told .Since he has gotten with her he has been very distant from me, and he stares at me. Every one says that she is going to cheat on him because she does it to all her boyfriends she has had. I need a lot of help here can someone please help me