Ok i like this girl but i dont know what to do.?

that's always a trick situation because she can just view you as her ex's friend or her friend and not consider you that way which will surprise her if u tell her. and she has a boyfriend. If there doing bad, u probably could ease in and show her how much u care which could make her fall for you. but if their relationship is steady, you have no chance and should leave it be. its just not in your cards.