First, I would like to establish that I served as an enlisted member of the USN. I retired as a Chief Petty Officer, and one of my responsibilities while on active duty was to assist newly promoted Ensigns during their transition to Fleet duties after they graduated from college or the Naval Academy. I am also by no means a U.S.N. recruiter, so I don't want you to misunderstand that point as it is a very important one.
When you are in your Senior, or maybe even Junior year in High School you will need to make sure you take the ASVAB (if its still called that) but it is a skills battery test that the military uses to determine where you would best fit into the service. Because appointment to the U.S. Naval Academy is an extremely competitive venture, you will need to make sure you are at or near the top of your class among other things that will help you reach that competitive level.
Jay recommends the following next steps:
Adding to Jay's comments, in order to receive an offer of appointment to the Naval Academy, an applicant must obtain a nomination from an official source. There are many nomination sources and applicants are encouraged to apply to all available sources. This normally includes a U.S. Representative, two U.S. Senators and the Vice President of the United States.
Billy recommends the following next steps: