Make sure you dress nice and be on time would be my first suggestion. Very important.
Job interviews are always nerve racking. I have one on Tuesday actually. To relax try not to put to much pressure on yourself. A lot of things have to go right to get a job. If they interview 10 people and you do great and come in second you still don't get the job. So there is a lot you cannot control. Just do your best it is good to get the experience under your belt no matter how it ends up. What type of job is it? Good luck