Meet her at her locker or whatever you school has ( being alone just you too helps, or bring 1 of your friends) and ask her in a clever way. dont just say wanna go out. a First impression lasts a lifetime.
If you are scared then there is nothing bad with that. If you want to ask her out ask her out while she isn't around a lot of people because if she rejects you, you will look like a fool. Ask her out when nobody is around you and if she says no get over it. By the way if she says no and your afraid she'll tell people ask to not tell anyone. Hope she says yes to you.