First give us an idea of what your diet currently consists of, as well as your current activity level. It looks like you quit working out after 8th grade, but do you participate in any sports?
Have you done anything in the past few months?
Do you have an MMA club you plan on joining?
--If not, how do you plan on training? (solo, friends, other activities, etc.)
Any sport requires dedication, but it is my strong belief that MMA requires serious dedication if you plan on competing. The best thing about high school is that you have access to sports teams such as wrestling, boxing, track, and other sports that would be helpful for your future as an MMA fighter. Since you're 17, I assume you are about to graduate? If you're a junior, plan on getting into sports before it's too late. Sometimes people think they want to get into a sport, and they just have to feel it out for themselves. You are young and you have a lot of time to figure that out. My suggestion is that there should be no question of whether you "should" do this, or that. Just freakin go for it. Take advantage of what there is at school, even if it's not combat related. That's why I mentioned track. Swimming and track I believe are the best non-combat sports just to get into shape. If you start training in MMA, just remember you will NEVER stop learning, or stop making mistakes. We can always get better, so don't get frustrated.
I meant to just ask a few questions, but kinda went on a tangent. Anyway, answer the above questions. They'll help us give you some good feedback. And good luck buddy.