If you are planning on applying to a service academy reach out to your guidance counselor and college advisor as early as possible! Spring of your junior year is preferred.
The United States Military Academy (USMA) in West Point, New York.
The United States Naval Academy (USNA) in Annapolis, Maryland.
The United States Air Force Academy (USAFA) in Colorado Springs, Colorado.
The United States Coast Guard Academy (USCGA) in New London, Connecticut.
The United States Merchant Marine Academy (USMMA) in Kings Point, New York.
For most of the Academies, you will need a congressional nomination, which you can find out more about here.