Girls opinions.......?

Can you try to put yourself in my situation? I have know this girl since we were literally in diapers. Now I am 13 and she is 14, we are separated by about 6 months. Now I think I have a crush on her. I was thinking about asking her to see a movie. But if she says no or she doesn't want to, I want to know how you guys would get back to being friends if this happened to you. How long do you think it would take before it would stop being awkward? If she said yes. How long do you think it would last (that one might vary)? Mostly what i'm getting at is, if you were in her position, what would you do?