"Hey, umm Terissa, like, wanna hang out sumtime?"
Hello there, I'm Donnie
It's intemidating trying to ask somebody to go out with you. But, just remember, if you don't do it but really wanted to do it, you'll regret it more than if you got turned down.
The thing that's most people's block is asking the person. They just freeze up! Puke it if you have to, in order to get a chance.
Try to catch her at her locker or something, and ask her out. Just say will ya go out with me, or something like "You wanna go get an ice cream after school?"
It's really that easy XD Just catch her at a time that you know she's in a good mood, and when she's not around alot of her friends. It might embarass her more than you!
And you shouldn't worry about not having a girlfriend yet, it takes time for some people.
Good luck, feel free to ask more