The above is great advise if you are looking to take it that far.
If you are joining just for the purpose of exercise you may not need to discuss MMA.
I am the type that studied martial arts for the other benefits. Self defense is definitely a worthy reason to learn. I did it more so for the body and mind aspect. I wanted to be in shape, be limber and also be more focus and dedicated.
Martial Arts lends itself to far more in life than self defense and competition events. It is a great way to improve yourself professionally. You will learn to focus and think quickly and clearly. It will teach you to be more dedicated and learn that the restraints you feel everyday are restraints you put on yourself. With the right amount of dedication and confidence, you can go places you never thought were possible.
Best of luck to you, I truly hope you find a respectable instructor who preaches these values and you take them seriously. It is only going to make you a better you.