Talk to her. You can't start a relationship without face to face conversation.
I want to ask this girl out. We both like each other. The problem is she is my first real crush and I have no idea how to ask her out or what to do if she says yes. She doesn't even know I like her back. What do I do?
Just do it
It is this simple...
You: Hey ____. I was wondering if you would like to go _____ (get a coffee, to a movie, got to a school event/sport)
Her: That sounds like fun. Sure I would like to go.