Do have problems with your best friend because you both like the same guy? Stop fighting! Your friendship is more important than a boy!
Talk to your friend. Tell her how you feel about the boy and that you want to go out with him. It's best to always tell the truth to your friend. You're good friends and no boy can come between you. Remember, this is a crush, not a competition. Also, the boy has his opinion too. Maybe he's interested in someone else.
Also, you need to have a simple understanding. Whatever happens and whoever of you two ends up with the boy, you will respect the relationship and not get mad. If he doesn't like you and does like your friend, be happy for her! There's another boy out there for you.