Whenever I go on an interview, I set up a mock interview with a friend and we go over questions that might come up. Whether they are the exact questions doesn't matter. It is the time spent practicing for the interview that helps.
You want to get over those initial jitters and not knowing what to say in the mock interview. Do this a few times and you will be more confident when you actually go for the interview. I call it getting into the interview mode.
What you really want to convey in an interview is confidence: confidence in what you know, confidence that you can do the job, confidence in conveying that information to the interviewers.
Don't forget to interview them too. You want to be sure this is the job for you. Sometimes we are so anxious about the interview, we really need the job or we think we have to say yes if it is offered to us, that we forget to notice the warning signs that this might not be the job for you.
And finally, you should do research on the company where you are applying. I asked one person I was interviewing if they had heard of our company and she said that she got an email alert that there was a position and she immediately applied for it. She didn't really know anything about us. Even after she told us this, she didn't ask any questions about the company. An employer likes to know that you are interested in what they are doing.