You're afraid to say something that'll be you look stupid or make things awkward. You GOT to have confidence when you talk to a girl you like. What I advise is to remember if you don't succeed, you can always try again with someone different. So talk to her like you got nothing to lose, coz you really don't have anything to lose. If you are already friends with her, do not be all overdramatic about it so you two have a better chance to remain friends. Hope this helps and good luck!
Its normal with growing up we all went through that stage of social awkwardness (I'm still dealing with it lol) but we all eventually grow out of it (atleast most of us lol I can't say much for my brother jk) just say hi and see how the conversation goes if you feel uncomfortable, excuse yourself but don't tell her she's ther reason you are uncomfortable....that would be rude lol
You probably have no self confidence, either because no one encouraged it in you at an earlier age or bullying has chipped it away. Just remember that the girl didnt say no, you didnt ask her anything :). If she rejects you, then she was a beach anyway. Keep that state of mind for at least a week and it'll stick with you.
You're nervous because you're worried that you might look silly or it's so important not to say the wrong thing that you get choked up. Just think of them in their underwear. ... Wait that might not help. Just think of them as people like anyone else. I used to be nervous around women. It's pretty normal but it can be a bummer. You'll probably get over it with time and practice.