What is your ultimate / primary desire? To serve in the military (any capacity)? To serve as a USN or USMC officer? To attend USNA?
There are various paths to achieve each. As a general rule (there are some exceptions), if you are on the college track in h.s., and want to serve in the military, your best bet is to apply to the SAs, apply to ROTC and, if neither of those works out, apply to Officer Candidate School upon completion of college. Any one of those will lead to a commission as an officer.
Most folks who enlist don't see themselves as attending college right after h.s. for all sorts of reasons (e.g., not good at school, don't like to study, want to work before more school, etc.). [Note: many who enlist ultimately earn their AA or bachelor's degree while in the military]. Most folks who enlist also want to be more of a "doer" and less of a manager (though senior enlisted have a lot of management responsibility). It's a different lifestyle that generally appeals to different people than those who seek to become an officer.
There are paths in the USN and the USMC to move from enlisted to officer; however, it's the exception not the rule and involves a lot of things outside of your control.
If your primary desire is to attend USNA, you have 3 more years to make that happen. Read the USNA.edu site in detail. Peruse these forums. Go to SA events held by your school, local congressperson, local USNA chapter, etc.