I like this girl but I don't know what to say to her or how to ask her out

What should I do?

Talk to her casually... about anything. Just as if you'd talk to a friend.

Don't ask her out right away.

Get to know her first!

Be yourself.

Uhh I really hope you know what you're getting yourself into.

First of all, don't think about it. If you do, you'll overthink it and make yourself miserable. Remember, the harder you think about it beforehand, the harder it's going to be to actually do it.

Just talk to her like you would anybody else. When you've done that a few times, then ask her out.

Once you've decided to ask her out, just do it. Don't wait for the right time, don't second guess yourself, and don't plan anything about the where or when (of the asking). Have an idea for where you want to take her, and include that in the asking. Doing 'something sometime' isn't going to be thought of as well as 'a movie on Saturday.'