Assume the interview starts right when you walk in. Conduct yourself in a professional manner because there may be people watching even when you don't know it.
What's the job for? Anyway, an interviewer knows that interviewee will be nervous. Don't worry about that part. The start of the interview may just be a little chit chat to relax you. Take it that way. It goes without saying that you should dress up for the occasion. It's not a problem if you look too formal, but it's a big 'no no' if you look too informal. Make sure you speak to the interviewer. Look at them in the eyes. Looking somewhere else when answering gives the impression of lying. Give your full attention to them when they are speaking, but don't feel embarrassed to ask for a question to be repeated or clarified. One final thing, have some questions prepared that you wish to ask them. There's nothing worse for an interviewer to ask if the interviewee has any questions and they say no. They don't need to be anything complicated, but should be things you're genuinely wanting to know.
Just try to seem confident in the things that you do and did. Name positive things about yourself and be honest about the negative things telling them that you work on them or don't. And being nervous is normal. Once you're there and see what you have in front of you it'll be fine. Just answer the questions and be honest with them and yourself. Good luck!