First off, starting high school and not having a girlfriend beforehand is FAR from bad. In fact, it probably makes you a better person, since you dont feel obligated to get a girlfriend in order to validate your own existance.
I'd say just try to loosen up.
Maybe if her friends knew you more, they wouldnt dislike you as much? If all they have to say is "Hi" then awkwardly leave, doesnt seem like they're succeeding, or even trying, to get to know you. Hang around with her more often. If she asks you if you want to come along after school, or just have the two of you hang out, then either her friends are warming up a bit, or she knows that her friends dont really like you, but wants to hang out with you regardless.
Oh, and dont worry about saying the wrong thing. Say what you would if it was one of your friends. If you try to be someone different, then you're going to have to stay that person your whole life, otherwise if things do work out between you two, the person she likes might not be the person you turn into. I say that from experience. Assuming that you want to, be the person that she likes.