The above contains good advice. To reinforce some of the specifics to the USNA, I would 1) apply for Summer Seminar on February 1, 2012; and 2) start practicing and taking the SAT and/or ACT. Have a copy of your transcript handy to complete your NASS application if you plan on turning it around in 24 hours. And remember, USNA only looks at your best math and English test scores, so practice accordingly.
You know the rest: get good grades, take Honors and AP classes and accelerated math and science, play sports, assume leadership positions in your extracurriculars, etc. My DS is a senior waiting for a nomination and just went through the process - Good luck!