First of all, you do not need to be 18 to train in MMA. Yes, you need to be 18 to fight/participate in an actual MMA bout, but training has no limitations. I am turning 17 years old this year and I have been training in MMA for some time now. I am not an MMA expert, but I have been doing Martial Arts for about 10 years now. Anyways, in MMA lessons, it can vary with different schools. However, the standard types of Martial Arts to be taught in MMA would be Muay Thai, Kickboxing, and Jiu-jitsu/grappling/judo.
Muay Thai would be taught to help you with your clinch, how to get your opponent in an effective and controlling clinch, how to defend and get out of a clinch, use effective elbows, knees, and other striking, etc. You can also be taught powerful leg kicks and inside kicks.
Kickboxing would be taught to help you use your reach advantage or to stay away from your opponent and keep them at bay. This would be done by using your jab, power punches, sidekicks, front kicks, roundhouse kicks, etc to attack your opponent from afar to keep the distance. Boxing would be for closing in the distance and just shooting short uppercuts and hooks from the inside.
Grappling, Jiu-jitsu, and/or judo are different types to learn to improve your ground game. This will incorporate throws, takedowns, submissions, chokes, armbars, controlling positions, etc.
These would be all the possibilities that you can learn from an MMA class. However, it is not limited to, and yours may not contain all of these. But here are some things to keep in mind and what to expect. Also, a few conditioning drills would be good.
Hope this helps, good luck.