I need advice on how to get a girl?

Honestly, I think trying to sabotage someone's love life is a little bit low, no offense. What I would do is wait until the girl breaks up with her boyfriend, or vise virsa (boyfriend breaks up with girl). Once the girl is single, for one reason or another, try sweet-talking to her.
Women have many things they love. And many things they hate. One almost all women love is flowers. It's a natural fact that most women love flowers. They do also love jewlrey, but I don't want you going bankrupt over one girl.
If you're broke and can't afford to buy her anything, try either sweet-talking.
If you prefer a more reliable choice, try to get her to like you back slowly. Become the girl's friend. Let you and her get to know each other a bit. Soon, slowly start to open up to her. Don't go telling her your entire life story on the second day. Around after a week, try talking a bit more to her about your life, she just might open up as well. Eventually, maybe after a couple of months, let your confession out. Admit to her that you like her and you'd love to be her boyfriend. She just might say yes.
I hope this helps you out. Good luck to you.

Be like Keith Washington