Throw up, if it makes you feel relaxed. But not too often. Much better to rub your stomach and watch something or listen to your favourite music. As I said, the most important thing for you is telling yourself that doing it at all is already a giant step. A perfect interview can happen, but must not happen.
Oh, and when it's about what to expect - I think you will find out only when you sit there. Be friendly - that's very important. And if you're shy or nervous, admit it. It's much better than panicking or freezing. Dress in something that makes you feel comfortable. And try to remember to look the interviewer in the eye a lot. That's always a good thing to remember.