Getting a good girlfriend in high school?

im 15 year old male in sidney ohio wanting a good girlfriend. im not very active, in school its like i don't even exist, im heavy set, but i love to have fun. there is one girl in my grade that i like alot but i think she doesnt even know im alive. im very chicken s*** when asking out a girl. i think she has a boyfriend but i havent been able to figure that out yet.i really like her and would like to go out with her. please anyone give me some good tips please.