Ryan it is nice to see such enthusiasm! I do hope you consider EMS as a potential career, and with that I will offer my advice from one that started in EMS at your age.
First let me state, please remember EMS will always be there for you to enter. As well, the later you enter the more stable and better it will be. The likelihood of finding a position as an EMT or even a Paramedic under the age of 21 is very hard. Also, you are brighter than many of your age, but I am sure you realize mental maturity takes time and life experience to develop. Yes, almost anyone can recite what to do and pass the written and skill test, but like medical school that is only a small part of the job. If you take an EMT course, the chance of you having to re-register before you even find a job is a high probability. Then you will become discouraged, then we will loose a potentially good person.
I highly recommend that take some science courses while in high school such as Advanced Biology, Chemistry, etc. You mentioned your an average student, so obtaining a tutor would not be a bad idea. Now the most important part.... HAVE SOME FUN! Really, your only a kid once and this job can suck the life out of you! So, enjoy life as much as possible, as many others can attest it will never be that way again.
Take your general education at a Junior College, no matter what profession you choose, you will always have it. This will even help if you plan to attend a University.
Military is great for some and then not for some. It all depends upon what you plan to do and what they plan for you to do. Please know military medics are really great for what they do and I have the utmost respect for them, but it MUCH different in civilian EMS. Cross training can be done, but not always a success.
I highly suggest to get a guidance counselor no matter what. They can steer you into the right direction, assist in scholarships and loans, as well interacting with college counselors.
Again, relax you have plenty of time. Please, please have fun and enjoy life. Forget about EMS for right now, and come back to it, it will be ready for you!
Good luck,
R/r 911