Take your time. She's not like any other girl, and she probably tells you every single one of her secrets. If you surprise her...she might be really embarrassed. Perhaps she just wants you to be your friend.
It's very hard to keep your feelings inside when you're young, so you might given her a lot of signs about that long ago. (She might even know about it and just be hiding it.
When, if, you decide to tell her, say something about it being okay if you just want to be friends, that you can get over it if she doesn't feel that way. Tell her you couldn't keep it to yourself any longer but you can deal with it if she doesn't feel the same way.
If you have that kind of open friendship where you also discuss your love lives and she asks you if you have anyone on your mind lately or she asks you about a girl you previously dated or that you show interest in, then casually say "No, not really just you. I've been thinking about how hot you are." and then laugh and say you're kidding. Little jokes like that lighten the mood and can make it easier to proceed to talk about this topic. After you make some kind of joke like this, try to tell her that you have no one in mind and your open to finding anyone new.(hint hint) ;)
if you are going to tell her your feelings talk to her personally, face-to-face, not in texts, or e-mail. Usually don't tell her that in public, because she might get very embarrassed. Don't do it drunk, either because that will always end in tears. It will also be very awkward the next morning, especially if neither of you remember.
Now watch how she responds. Does she try to keep the conversation as short as possible or does she talk more or less openly?
If she doesn't end up liking you back, stay cool. Be casual and try not to let the friendship get awkward. Try to get things back to the way they were before.
