This is clearly a highly subjective subject, and my own opinions are obviously biased towards personal experience, but I would strongly dissuade you from trying to take "MMA" as your first martial art/combat sport. The sheer breadth and scope of techniques, positions, and possibilities in mixed martial arts makes it very difficult to just pick up without a solid foundation in one of its four core disciplines (bjj, wrestling, muay thai, and boxing- obviously other bases exist such as judo, kickboxing, sanshou, etc, but for simplicity's sake focusing on the big four). Though it was not on your list (and therefore might not appeal to you personally), if you are in middle or high school I would highly suggest joining the wrestling team for the following reasons.
1. It's free.
2. You are more likely to get quality instruction from a knowledgeable coach who has a vested interest in your success. There are a lot of Bull**** artists out there teaching "martial arts", and many of them will look to take money from a young student who might not know any better.
3.You will be part of a new social group that might bring lasting friendships and a foundation of support throughout a period of life that most people agree can really suck.
4. Finally, and least importantly, if you eventually do look to train in mixed martial arts, you will have a solid foundation of skills and athletic, competitive experience.
If striking is your desired focus, then I would suggest you research local boxing gyms and their instructors. It can be kind of hard to find a kickboxing school depending on where you are, but you can certainly find plenty of kickboxing instructors working out of boxing gyms, so start your search there.
Remember, it will be the instructor, not the style, that will be more important at your age.
