It is best to talk to your local recruiter for the most recent policies - but, most military branches have a 'delayed entry' program. This program allows you to sign up when you turn 18 (or in some cases when you are 17 with parents consent) but you don’t leave for basic training until a predetermined date after you graduate from High School. Note - most branches are very competitive these days and will not accept enlistees without a High School diploma. While in the delayed entry program you are required to check in with your recruiter regularly, and there may be events that you must attend to remain in good standing. Good luck and thank you for considering serving your country!