Feel good about yourself and remember that you are there because your potential employer has seen something in you that would assist the companys interests. So this is a reason to feel confident already. As long as you know what you have written in your resume, and everything reflects the truth, you dont have to feel insecure. Now its your chance to prove to them that they made the right decision to call you for an interview and as a consequence to hire you. Learn to market yourself and remember that your product to promote and sell is you. And what is the best way to market a product? Believing in its value first! So believe in your own value as professional and support it with confidence.