You just talk to her as a friend. Don't talk to her with the intention of stealing her away from her boyfriend. Just be friends with her as you would be friends with anyone. You're learning how to...
They won't stay together long, if they have a messy break up you can be there to comfort her and there is a good chance she will hook up with you. Or you can spend time with her, appear sweet,...
first thing don't freak out. these things are very common at teenage. try to know indirectly whether how she treats you (as a "just friend",best friend or more than that.) if she likes u then u can...