Seeming nervous on a job interview can definitely hurt your chances of getting the post. Although there are many reasons why people get nervous, lack of confidence is often a major contributor. Preparing for a job interview is extremely important, and it's probably the best way to combat nervousness in an interview.
You should be keenly aware of your own strengths and weaknesses. Have specific examples in mind of contributions you have made. If you want to gain more experience in a specific area ...