First, congratulations on an interview! Best wishes to you. Depending on the type of job it is, you may want to be upfront about being nervous. It shows honesty. If possible, take notes during the interview to help you stay focused. I have written the word "breathe" on my note sheet to help me remember to relax. I also like to make sure I arrive with plenty of time and I bring something to help me relax if I arrive early (relaxing music for example).

Best of luck on your interview!