Dress smartly; shirt, trousers, shoes, etc. Appearances shouldn't matter, but they do. If you go in there wearing casual clothes then you're going to be giving off the impression that you aren't really bothered about getting the job. First impressions are everything. If you look good, it'll help you feel good.
Do your research about the company. What do they do? Who are their customers? What is the job likely to involve? How can you best fit your skills to match the job?
Questions to ask if they give you the chance to do so are:
What are the most and least enjoyable aspects of the role?
Is there a chance of a future promotion?
How would you describe the work culture here?
What is a typical day like for someone in a similar position to the one I am applying for?
How will you measure my performance? How often?
Asking questions when/if the interviewer says "Is there anything you'd like to ask?" is important. It shows you've thought about the job and it shows you're enthusiastic about the job role/working for the company.
It's also important to prepare for what the interviewer will ask you.
Why do you want the job?
Describe a situation where you worked in a team?
What do you expect to be doing in five years time?
What are your strengths/weaknesses?
Who else have you applied to/got/had interviews with?
Have you done this kind of work before?
What do you do in your sparetime?
What maks you think you are the right person for the job?
Make sure to practice your questions and answers reguarly beforehand.
Also, know where you're beng interviewed and how you're going to get there/how long it takes. Make sure to arrive 10/15 minutes before your interview is supposed to begin. And be sure to have all the relevant paperwork with you; CV, qualifications, etc.
And remember if you're nervous when you get there that it's natural. If you're nervous, your voice might sound shaky and squeaky. Practice slow, deep breathing before you get to the interview, it'll slow your heart rate down and help you avoid taking quick, shallow breaths.
Enter the interview confidently. Shake hands firmly and introduce yourself. Always smile. Be polite, friendly and look the interviewer in the eye at all times. Check if it's okay to use cue cards/notes during the interview (you might want to write some questions to ask down). Look interested and ask questions as well as answer them. Answer questions as fully as you can, don't just say "yes" or "no". Provide examples to prove your skills ans achievements. If you don't understand a question then ask. Speak clearly. Sell yourself, get your good points across and be positive. Listen. Answer questions with examples. Be prepared.
Don't treat the interview as if it's "just a job" at some fast food joint. Treat every job interview the same. Best of luck!