Listen pretty simple
Be yourself, by that I mean don't try to impress her and don't act weird or funny. Talk to the girl and be nice to her. Start knowing her, she might also be pretty but how is she in the inside? After knowing each other tell her that you like someone and for a few tips. If she goes red (blushes) that means she likes you
I am in fifth and i already got the hottest girl in town as my gf! You dont need to be older than fifth to get a gf.