Take Junior ROTC in high school to first see if you're cut out for some of the regimen that is required in the military. Once you get into college you should understand better your needs and capabilities if ROTC is for you.
Completing ROTC in college should assure you at receiving a commission in the U.S. Army as a second lieutenant. You could then go on to Airborne training and then to Ranger school--provided your physical attributes can qualify for such.