Help with this girl who has a boyfriend..!?

Act just like a best friend. You know she has a BF already, so don't go past your boundaries. If she comes onto you, then that isn't very good GF material. That would be called cheating, and you wouldn't want a GF that you've seen cheating would you? Cuz you never know (if you guys do become official) if she'll be cheating behind your back. So unless she has broken up with her BF, then I would say it's safe to confess your feelings to her