15 and in grade 11? Okay, well you lied about your age but anyway ..
Just talk to her. If you haven't met her yet introduce yourself to her and say hi to her when you see her. Get to know her and compliment her when she looks nice. Don't treat her too nicely and don't make fun of her too much.
Remember, talking to a girl is not that much different from talking to a guy. Think ; if you were a girl, what would be the type of guy that you would go for?