It's said that you never get a second chance to make a good first impression, and this is ESPECIALLY true on a job interview. Dress to the nines, but fail to understand what the hiring company does, or why your skills meet their needs, and you might as well have stayed home. If you can't articulate why you would make a better candidate then all the other applicants who have put in resumes, you're libel to walk away empty handed, never to hear from that hiring company again.
Know what the business does. Know what you bring to the table, and understand how your skills will allow you to perform your job at a higher then average level. Be professional! (you'd be surprised how often this gets overlooked by first time job seekers) Look and act the part. A certain level of nervousness is normal and, in many cases, expected. But, don't let that nervousness overwhelm you. Taking part in a mock interview beforehand helps you to anticipate questions, so you can have ready ideas and answers available when those same questions come up in the real interview.
Prove to that interviewer that you're the best person for the job, and you just might land it!