The best way to prepare is by not preparing. Reading over interview questions and planning your response will only hurt you in the long run. Your answers will sound disingenuous and rehearsed. You want your answers to flow naturally during the interview. It is better to have a small pause before answering, than to jump at each question with an answer. It is also better to have some ideas flowing in your head about what you want to talk about, or how you would answer some tricky questions; but only ideas and not answers to these questions. It is beneficial to arrive early to the location and breathe for a while. If you look too nervous during the interview, it could look like you are hiding something. Some anxiety is expected, but you should not be shaking the whole time.
These are some programs that are popular among high school students. This is by no means an exhaustive list of nationally recognized programs. Boy’s/Girls States