I really like this girl and I know she likes me because I sent her a note. She sent it back saying she liked me. Basically, I've done all the work upfront, I'm just not sure how to ask her out. If it helps, I'm 13.

How close are you two in the first place?

If you are already friends or at least, an acquaintance, then horray! Half of the work has already been done. Now all you need to do is to ask her out. The key to this is to be subtle as in making sure that you bring along mutual friends and tell her that it's a "group" thing. This will take off some of the pressure. Go watch a movie and sit next to her while in the cinema. Then once you're done, go have a snack at Maccas or something and talk to her while eating. That way you are still hanging out with her out of school but it is not an official "date" so there will be no need for awkwardness. And if you run out of things to talk about, you have friends to back you up.