Your question should be "What should I do to improve my chances of getting into the USNA". SS is an opportunity to get a close look at USNA and gain additional insight about the Academy. In USNA's words "Summer Seminar teaches you about life at the Naval Academy, where academics, athletics, and professional training play equally important roles in developing our nation's leaders." You should familiarize yourself with the USNA website (i.e. https://www.usna.edu/Admissions/Steps-for-Admission/General-Advice-for-Grades-9-12.php ). Read the "sticky threads". Stickies provide useful information and guidance. You can certainly start training for the CFA. Muscle growth doesn't happen overnight. Make sure you plan academic schedule to best position yourself to be competitive. Sport camps are a great way to visit the Academy to.
The tongue in cheek answer is "move to, or be part of, one of USNA's under served demographics." NASS is a "outreach" or "awareness" program, intended to promote awareness of the Naval Academy and its mission. It is NOT a prescreen or qualifier for admissions, and getting selected or not getting selected for NASS is not an indication of your competitiveness.
If you are interested because you want to see whether you like USNA or not, a CVW gives you a much more realistic view of Academy life. That being said, attending NASS is nice because it gives you a chance to complete your CFA and meet some of your future classmates. The NASS cadre also writes an eval/recommendation, but I question what weight this is given . (Purely spectulation, but would expect that the cadre input has little effect except at extreme ends of the spectrum).