I'm going through the same thing, had my first real grown up job interview today! Couldn't sleep last night, couldn't eat breakfast, got outside and wanted to bottle it, my hands were shaking and everything, I just wanted to run home.

As it is once I got in there I really did calm down. Try and see it as a two way question and answer session, after all you're going to be working for them so it's important you get chance to ask them things as well. Trust me, as nervous as you are, after a few seconds you will calm down and fall into your rhythm.

Hope you have better luck than me though, I sucked! Didn't really have the technical knowledge they were after so i couldn't answer most the questions. And Just keep telling yourself if you really do bomb and make a tit out of yourself, you'll never see those people again so who cares? go for it, you have nothing to loose!

Oh and one thing I did that really helped, talk to yourself, sounds crazy I know, but stand in front of a mirror and ask yourself some typical questions (these are some of the ones i got today)

So tell me about yourself?

What's your biggest weakness?

Why do you want to work for us?

What would you consider your biggest achievement to be?

If a customer got aggressive and became violent, what would you do?

Say the answers out loud, they always sound different in your head, and pay heed to your body language. Good luck.

Before you go in, make sure you masturbate.

I almost freaked out, but I kept it in check on time and got the job.