[edit] Steps

First, make sure he doesn't have a girlfriend.
Decide if you want it to be a group thing or a just the two of you thing.
Make sure when you ask him you look good and have nice breath.
Don't come on too strong and make him think it's a date, unless he likes you too.
Do a little pre-flirting before you ask.
Now for the asking part... act totally calm like it's no big deal. If he knows you like him and he doesn't like you back, make it a group thing. You don't want it to seem like a date yet.
Just casually bring it up like, "Hey, my friends and I are going to the movies Saturday. We're watching ________. We're trying to get as many people as we can.." Then later in the conversation say, "Hey, why don't you come?" or maybe even "Maybe you can grab one of your friends and I'll get a couple of mine and we can all hit the movies on ________"

[edit] Tips

If you're nervous and don't want him to know that you like him, it will be better if it's a group thing.
Be prepared for anything.
If he says he can't make it, don't be upset.
You should do a group thing the first time, he is probably as nervous as you are.