Wow...that's a loaded question.
First, you can enlist in the National Guard when you are 17, do you Basic Training during the summer before your senior year, and drill one weekend a month with your Guard unit during your senior year. That, however, will not necessarily put you on the path toward being a helicopter pilot.
If you want to fly rotary wing in the Army, you can apply for the Warrant Officer program, in which you will need an Associates Degree.
To fly rotary wing in the Air Force or Navy you need a Bachelors Degree and a commission as an officer.
All those banches have medivac squadrons/battalions that you get into and train for. Also, the Coast Guard has similar programs, also requiring a Bachelors Degree.
You do not have to have 20/20 vision as other in here have stated. The requirement is 20/70, correctable to 20/20 with glasses or contacts, with no astigmatism (that's also true for fighter pilots).
I know in the Air Force, once you complete helicopter training, you have a 10 year active duty service commitment to fulfill. Once done with that, then you can separate from the military and pursue your dreams elsewhere (still serving at least 3 years in the Inactive Ready Reserve).
Lastly, if you want to be an EMS pilot in the civilian world but not one in the militray, then how will you get the hours and experience necessary to get hired in the field when you get out of the military? Being an EMS pilot is a highly sought-after and highly-selective field. Only those with thousands of flight-hours and top qualifications are hired. You can't join the militay as, say an infantryman, fly helos during your off time, and expect to get the hours you need to be competitive in the civilian market.