This is my take on the selection process.
The boards are looking for someone who shows the potential not only to perform well academically and physically, but someone who is a well rounded individual and who has the potential to make a positive contribution to the officer corps.
Your first and foremost focus should be on maintaining your grades, while at the same time taking challenging classes. It's good that you're already starting to take AP courses, now I would encourage you to take as many AP courses that you can safely manage in order to challenge yourself. Don't let your GPA fall below a 3.5, but see if you can get it up to a 3.7 or so. SAT and ACT scores are a factor as well, and are judged as your cumulative best score (for example, your best SAT math score from your first exam would be paired with your best reading score from your reading comprehension from the second time you took it, etc.)
Like I said, make sure you're well rounded as well. The goal is here is they're looking for indication that you'll be able to balance a healthy social college life with the demands of ROTC and academics. Sports, band, scouts (scouts are a plus, especially if you're an eagle), martial arts, committed hobbies, etc. At the same time, the selection boards like to see someone who is committed to their extra curricular. So you neither want to be completely top heavy in one area (like all sports and nothing else), but at the same time you don't want to be a jack of all trades and master at none. Being All-State in band is good, make sure to mention it as well as another achievements you have in your extra curricular.
When you show up for your interview, show up looking professional, but don't show up looking like a tool either. I would recommend slacks, white undershirt, polo shirt, belt. Make sure you're well groomed with a fresh haircut. Each recruiting station Executive Officer gets a couple of scholarships he can hand out personally on the spot (known as Immediate Selection Decisions). If you impress the LT enough, you could potentially qualify for this. Don't ask about it, though. If you get it, cool. If not, you can do the selection boards with everyone else.
Edit:
I'm looking at the ISD checklist right now. Of the following three items, you need to be heavily involved in two of them; Extra-curriculars, athletics, leadership.
I forgot to mention. Leadership positions wherever you can get them, whether they're in sports, club organizations, wherever. Very big factor in both the ISD and selection board routes. And show initiative.
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