How do I talk to someone?

This is the same girl I talked about a couple posts ago. She's a girl in my class that I kind of like. It's like there's an invisible wall preventing me from talking to her. I just loose all confidence when I'm about to talk to her. I'm afraid I'll mess Everything up I mean she probably doesn't even know I exist on this planet. I just get insanely nervous around her. I need help, what do I do?