Job interviews are always stressful - even for job seekers who have gone on countless interviews. The best way to reduce the stress is to be prepared. Take the time to review the "standard" interview questions you will most likely be asked. Also review sample answers to these typical interview questions.
Then take the time to research the company. That way you'll be ready with knowledgeable answers for the job interview questions that specifically relate to the company you are interviewing with.