Be preparared for common questions: Talk about a book you've read. What is favorite class and why? Talk about challenges you have overcome (outside of school). Have some questions to ask the interviewer that reflect the time you've taken to look at the website (something like, football's doing really well this year, do many students go to games....make sure football IS doing well!!).
Dress neatly, smile, make eye contact and shake their hand.
Practice the "Smile, handshake, hello" with a few adults.
The interviewer is not there to grill you. They are mainly checking that you are indeed likely to be the person who wrote the application, not your great aunt; that you can speak English and that you are not an obviously psychopathic axe murderer.
It helps to let them know you are passionate about learning, like school, have some friends, that you know something about their school and why you would be a good match there.