If I were you, I'd dress in "business casual" type clothes, eg Dress pants and shoes with a collared shirt.
Seeing as your under 16, I'd assume that your interview won't be too intense. They might ask you about school, previous work experience, hobbies, etc. They'll probably ask you to tell them "a little about yourself", so maybe think about your answer to that question so you can answer it confidently.
Is your interview an individual interview or a group interview? That might help determine what might happen.