I registered here just to answer the OPs question.
Here's how to ask a girl out. I'm not gonna give you childlike details. You've gotta figure out how to do some of it by yourself, but i'll give you an outline.
1. Arrange to hang out with her sometime. Not a date. Just you and her, hanging out. This shouldn't be hard to do. Be creative in how you manipulate circumstances in such a way that it ends up just being you and her. Just asking her is probably the best way. Ask her if she wants to hang out with you after school or something.
2. Repeat step 1 until you can manage to get her into your house, or get yourself into her house. Get to know her well enough that you feel comfortable around each other, but not well enough that you drift into the friend zone. This should all still be well within your comfort zone because you haven't said a word to her about asking her out or dating her.
3. After you've established these circumstances, wait until a time when just you and her are hanging out together, and tell her that there's something you have to ask her, but that you're scared to. She'll show interest, and find your shyness and insecurity attractive. Keep her on the hook for a while, even a few hours (i've done it). Tell her that you really want to ask her this question, but you're really scared. Try and start asking her but then fail to have the courage to bring the words out. Eventually, you can finally ask her something like "do you think i'm attractive?" Tell her that that's not your actual question, but that you chickened out. Based on her answer, you'll know whether you can ask her out or not. Finally, after she thinks you're adorable, and has repeatedly encouraged you to just ask her whatever it is you want to ask her, ask her if she'd like to go out with you. If she doesn't want to by this point, then it wasn't meant to be, you have been spared and there is nothing to be sad about.
4. Keep in mind, these steps can be mutated slightly depending on the reality of each persons situation, but the bottom line is, you've got to be one on one with her, be scared to ask her "something" and make sure that there's enough of a connection between you two that she shows interest and strongly encourages you to ask the question, and shows signs of liking you. It's that encounter when you finally get down to asking the question that is the ultimate goal of these steps. That's all you're working towards.