I applied for NROTC but I'm sure the interview is similar.
My advice would be to be yourself, yet stay very respectful. Take time to consider any questions you are asked. Look professional as well! Make eye contact, try not to fidget. The normal do and don't for any interview or speech, really! You'll probably be nervous, but that's okay! I'm sure they would be worried if you weren't nervous!
I was asked to name what AP and honors classes I had taken, make sure you designate which are which (say like AP Calculus, Honors Spanish, etc etc) if you are asked.
Also, if you are any clubs that have acronyms for their names, know what the acronym means. Like I'm in E.A.S.T. initiative - I had to look up what E.A.S.T. stood for during the interview. I don't think my officer cared much (we raced to see who could find it first) but I don't know if everyone would respond that same way (the officer I had my interview with was very cool).
