Hey!
I haven't been to an interview ever before, but from what I have gathered reading other's experiences and what not, the key is to be yourself. It's really important not to try and 'act' friendly and very social, because it just doesn't seem natural. Speaking nice and clearly, answering questions with confidence and interacting with others is really important, however don't force anything. It's also great to show that you're keen, such as by asking questions, but again don't overdo it and don't force it. If you are put into groups, like many selective school interviews are like, try to lead the group. What I mean is to be confident, come up with ideas, but also willingly listen to others . This will really show that you possess leadership qualities and that's one of the main things that the interviewers will look for in a student. 
They are pretty much the basics based on what I know, as I haven't been to an interview, ever. I really hope this helps you in some way, and I'm sure other users will be more than happy to help you out even further. 
Good luck!
Galactic, you did it :PP
I'm not good with emotional advice, but I can give you some practice interview questions.
• Why do you want to go to this school?
• What do you hope to gain from this school?
• What do you want to be in the future?
• What's your greatest strength and when have you demonstrated it?
• What's your greatest weakness and how are you overcoming it?
• What qualities are needed in a successful person?
• What hobbies do you have?
• What extracurricular activities have you participated in?
Glhf with your interview (rip, still waiting on mine ;-;)