How do you ask a girl out that might be out of you league?

theirs this girl at school that i like. im in high school and i want to ask her out. the problem is im afraid to ask her out. the first reason is i don't really know her well. not personally anyway. second, i get scared and chicken out at the last Minuit. third i don't know what to say or when to say it. do just go out and ask her or should i be her friends first. do i try to Waite until an emotional moment. i knead helllllp