Go see the counselors and take the batteries of free tests of apptitude & preferences
I knew a lot of med students, and pre-med students, who worked part-time ambulance as a basic EMT for pocket money. I was a moonlighting (during broad daylight) full time USAF firefighter/rescuemen working nearly full time on a private ambulance on my off days, and a "pre-nursing" student and new father when I worked another service of the same sort.
That said, know three things:
1. EMT is a skill set not necessarily a lifestyle. As a career it is truncated and offers less pay than you could expect elsewhere. You will eventually be physically unable to do the lifting, and the opportunities for administrative work with just an EMT certif are very very small. Having said that, being an active EMT can tell you about your reactions to patients and their personalities and problems, as well as an intro to anatomy etc.
2. The "medical field" does not have a central career ladder you start at in one field then cross to another; othe than some "bridging" programs, every time you change (say, from EMT to Nursing) you start over. If you want to be a RN, go for it, don't expect LVN or EMT to be "stepping stones". (Although, nursing students with some experience in the real world seeing patients and managing their time can have a better chance at good clinical grades).
3. The majority of baccalaureate graduates do not wind up in their chosen field. Of those who do get jobs in their chosen field, some are disillusioned (that is a high percentage in nursing, especially male grads) and of them some will quit, or just sit there and be miserable.
Suggestion:
1. Get Sr Lifesaver from ARC, and first aid training, this summer. Might get a nice summer job out of the deal too! Will also get you around people in a certain role. Maybe make some saves?!
2. See your counselors. Look into seeing college counselors if they will see HS students (some will).
3. In high school, do what you need generically to get into a good school and maybe earn some scholarships, and get into good shape physically (good for EMT, good if you go military, and generally makes you more ready).
Enjoy your summer!!!