The fear of messing up your job interview is a very real one, and if it goes unchecked it can turn into interview failure and ruin your chances at the job. Here are some tips courtesy of US News' Money section that should settle your anxieties before you go into that room.
1. Remember that this interview needs no more form you than you really are.
In other words, the only effort you need to put in is the one to remain stress-free. You are a professional, with skills and qualifications - or you wouldn't be here. But you are. You have a lot to offer, and while you shouldn't let that get to your head, don't forget that they would be lucky to have you. You are so much more than just a person who needs a job.
2. Don't forget that this interview was called by them.
So clearly, they think you might have something they want. They wanted you to be here. You've earned this interview, and now you just have to be yourself and show them they were right. Don't be intimidated, feel validated.
3. Remember that the only person who knows you completely is you.
This means you choose to show them what side of you they will see. No, this doesn't mean lying. But there's no reason they have to know about your insecurities. Make your case, and do it with focus and confidence.
4. Ask yourself: what is the worst case scenario here?
Answer yourself that and you will only end up not getting this job. Who knows? This job might not be fit for you anyway. You will move on, get more interviews, get more jobs. Life will go on. This is not the time to think about how you will face your friends at that lunch, or how you will be letting down your parents' expectations. They can all be dealt with. This is your life, and you are doing the best you can, and that should be enough. There will be other opportunities.
When you think like that, you lessen the negative effects. When the negative aspects are lessened, you turn even a bad interview into a learning experience. But most importantly, you won't be stressed anymore because you no longer have everything to lose.