just be yourself and maybe make yourself look good in front of her. also make sure she knows you like her, by telling her or making it obvious what ever because you never know she might like you
if you both like each other, then ask her out. (: most girls do not want to make the first move, and many wont. if you keep putting it off, she might find another guy and move on. and if she says no,...
Well, if you truly like her, you'll give it a chance. Girls don't like to have to wait on guys because they want freedom. So just ask her to go out with you because she won't wait forever.
You could...
Look for another girl. Sometimes rejection makes the girl you fancy even more attractive! However, as soon as you find a new girl that does like you, you will forget this one.
Figure out why she doesn't like you. Fix that problem. Talk to her, and convince her that you aren't so bad. If she just downright refuses, then move on..