How do I make the first move?

Basically, I've liked this girl in my youth group for about a year now. I think she likes me, and I'm pretty positive she does. I really want to ask her out, but because we're only 15/14, I always thought we were kinda young. This summer we have our usually camp which lasts for about a week, and the last night we have a banquet where you can bring a date. I want to be able to ask her to that (it's like July 4). But I'm so nervous that I'll ask her out or ask her if she likes me because I'm afraid she'll say no. How can I get to the point where I know she likes me so I can ask her out?