It depends really. But if a girl says no then you have done somethign wrong with your game.
It could be a shit test. She wants to know if you are really interested in her. In which case, you have to have the eye to know it.
I've posted a similar answer before because it is something that has worked for me and have been confirmed to work many times.
Do not ask a girl out until you've had some really good indicator of interest from her.
In your particular situation, she has said no, she knows you are interested in her already so it puts you at a disadvantage.
Don't ask her out again soon, let some time pass. During that time you have to show her a demonstration of higher value from a longer time perspective. This basically means you let her see you flirt with other girls, let her see that even without her you are fine.
When an opportunity crops up that you can chat her up again, don't just blurt it out, entertain her, prepare some material that she might be interested in, like movies or whatever she likes to talk about, prepare some games like "The Cube Test" google it. The Cube test is a personality test to get to know someone in a unique manner. Girls like guys who can entertain them.
Then after that make a few moves that stablishes if she is confident with you and you can touch her, play a palm reading game and arouse her touch senses, play with her hair by putting your arms around her, she'll get confused if you are really just wanting to touch her hair or you are going to pull her close and kiss her. I think you get the drift.
Then you can ask her out when you know for sure she will say yes anyways.