Be honest, if you don't know the answer to a question, simply say you don't know, or are not sure of the answer; you might tell them how you are going to find out the answer. If you don't feel comfortable about a question, stall for time, tell them you would want to really think about that particular question before you gave a full answer.
Do your homework, investigate the school on its website, and if you know any alumni, talk to them about their experiences.
Be polite to your interviewer. When you enter the room, put your hand out to shake theirs and say your name clearly and "How do you do?". No earbuds, and no cellphone; let me say that again, no earbuds and no cellphone. Make sure your cellphone is on silent; if it does ring or whatever, apologize and ignore it. Your interviewer is far more important than a text from your buddies. Sit up, do not slouch, and listen carefully. Look your interviewer in the eye, and think before you speak. Smile, Show some enthusiasm for the school. If you are nervous, it is OK to say so; this is a big deal.
Wear appropriate clothing. It does not have to be fancy, but what you wear should be clean and modest. A button down shirt and pants for the men should do. For women, a skirt or dress would be nice, but pants are fine; nothing revealing or too tight. Remember, you are there for an interview, not a date. However, use your own fashion sense, the bottom line is you want your character to be remembered, and not your clothing.
Also, after the interview, be sure to get the name of the interviewer (first and last name, spelled corrected) so that you can write them or email them a thank you note. Thank them for their time, tell them how much you like the school, and so on.
You might also want to do some role playing with your friends. Take turns asking each other about your longterm goals, why you are interested in a certain school, what your favorite subjects are, and so on.