I'm in love with a very good girl friend?

Ask her, do you like me?
Tell her her hair looks nice and compliment her and she might get the idea.

Speaking as a girl, I think if you were honest with her she'd appreciate it. Maybe you could write her a letter telling her about your feelings, so that instead of telling her personally and making it a little awkward for both of you, she can have time to think about it.
If writing a letter is too cheesy, then maybe start complimenting her a little bit, take it slow at first. If you decide to eventually tell her your feelings, make sure that she knows you don't want to ruin your friendship. Like I said before, she'll appreciate the honesty. Good luck :)