Do the right things in high school. Do not do drugs or alcohol. Study hard, stay away from distractions and substances, get into sports. Good grades are important, but good study habits are critical.
Choose your preferred branch, and get acquainted with a recruiter. If your guidance counselor's office doesn't have contact information, check your white pages under US Government, Department of Defense. Any recruiter will be thrilled to make sure you have all the forms, materials, and support you need to apply for a ROTC scholarship. They ask you to take a couple of tests. The better you do, the more they will want you.
Do as well as possible on your college entrance exams. This is the single most important criteria used to select high school candidates. Get a good night's sleep before the test, and have breakfast, no matter how nervous you may be. If your blood sugar gets too low, your brain will run on empty. Eat a sugary snack right before the SAT or ACT exam.
If your high school has a Junior ROTC unit, join it. This will give you a little advantage in college, although many many people have never worn a military uniform or walked together in a synchronized manner (they call it "marching") before entering a college ROTC program.
Keep up the physical fitness training! A physical fitness exam is not something you can cram for at the last minute! You should be able to run a good mile and a half in less than 15 minutes, do a dozen pull ups, and crank out 50 sit ups at least, or you will be doing a lot of remedial physical training.
Sign the acceptance letter when a ROTC unit chooses you, or pick which one you want if you get several offers. You pick two or three at the time of the college entrance exams, as well as the ROTC program you prefer. Your military branch makes your scores available to all university ROTC programs for that branch. So, even if your first choices pass on you, another university ROTC may step up to offer you a scholarship. Or...
Don't let it get you down if you don't get a scholarship offer. Go to a good university with an ROTC program, and enter as a no-contract candidate. It's HARD going to college AND meeting obligations as an ROTC member. MANY students end up giving up their scholarships during the first year. ROTC units look at no-contract ROTC students first. Get good grades in college, stay in shape, stay out of trouble (don't even speed!) and you have a great shot at getting that scholarship.