Get information on all the branches of service, meet with recruiters and get the information you need to decide which branch of service is for you.
If you plan to apply to a U.S. Military Service Academy, then start the process as early as you can.
Find out what you need to know to do well on the ASVAB and how the score is used to determine your eligibility for careers in the military.
Get the information you need if you are interested in participating in ROTC in college.