As someone said—you will be asked to talk about situations you’ve been in where you demonstrated various skills, abilities, or aspects of your personality. Start thinking about what those can be and practice telling them. Think about it from the STAR perspective: Situation, Task, Action, Result. You should be including all four in any story.
Don’t be afraid to take a moment and think before answering a question. A well thought out answer is always preferable to a jumble of words.
Do a mock interview. Get your mom or a friend to ask you questions they find online. Tape yourself. Watch the tape at least twice: once with sound and once without. Without sound you’ll really focus on things you do that are distracting or generally bad (nervous habits tend to come through here). If possible, do something that will actually make you nervous during this part (like get your family to watch or invite a guy you like over—whatever).
Answers should be long enough to answer the question, but answer it fairly succinctly.
Don’t forget to tell them you want the job. Ask when you should expect to hear from them again. Send a note afterwards thanking them for the interview. If you get out of the interview and think, “Oh, man, I really should have said XXXX”, include that in the note.
Get a good night’s sleep before. Eat breakfast unless that makes you queasy. Brush your teeth. Lay out your outfit the night before or in advance just so you don’t have to worry about it.
Be comfortable with information about the company. Make and maintain eye contact. Don’t say anything negative if you can avoid it.
Think of a few questions to ask the interviewer ahead of time that show you’ve researched the company. Write them down. Bring them with you. This way you can reference them if you like. Remember—you are interviewing them as much as they are interviewing you.