Know her better first. Go talk to her the next time u see her. Get her number or email and chat with her on sms or msn. But dont spend too much time just chatting with her. I did just that once then another guy koped her before i did anything significant....
soulness
something to add, and also don wait too long! girls don wait for u.. don be the silly guy who stand by her all this while without taking action stirke while the iron is hot