Its Okay to be Nervous
Knowing how to act confidently using body language can increase your chances of passing the interview. You can utilize this knowledge to conceal your anxiety a little, but this is something you shouldn't worry about too much. Many applicants are tensed during an interview, and they would not want to let the interviewer know about their inner feelings. However, it is completely understandable to be nervous at this stage. It is completely normal.
Your nervousness may even indicate how valuable getting this job is to you. If you weren't nervous, and you act like a happy-go-lucky person, you might be perceived as someone who is not very interested in the job.
The interview not only functions as a way of determining who among the applicants is most capable of performing the job well, but it is also a means of allowing the interviewer to get to know more about the applicants. It's a first encounter with an individual that you might soon work together with. If thats the case, then the interviewer (who could be your boss) should actually feel the same way as you are. Nervousness often accompanies excitement.