It all depends on you. Some people don't even take an MMA class. A lot of people take classes in specific disciplines. I was taking an MMA and a BJJ class for a bit (currently trying to find a new gym) and it worked for me. I also boxed for awhile.
MMA is a mixture of striking, wrestling, and submissions. An intro class would be beneficial if you are completely new to it.
As for working out, the training at the gym will be very good for you. Just make sure to try and stay away from empty carbs such as sweets and soda. They won't do you aby good. Protein will help a lot with working out. Make sure to do a lot of cardio-vascular exercises such as running and swimming. Remember that a lot of times the more muscle you pack on, the faster you will tire and the less flexible you will be in some cases.
I hope this helps and I hope others come along to contribute. Good luck, have fun and train hard!