You must start training with weights. Bodyweight exercises will not be of sufficient intensity to stimulate the type of muscular growth which will result in a significant gain in functional bodyweight, i.e. muscle and not fat.
Most MMA fighters are grossly overtrained. That's why for all the training they do, most haven't developed extremely muscular physiques. If you look at sprinters you will notice that invariably, they have very impressive muscular development in their legs. They train briefly and infrequently. The intensity of their training dictates that their training must be brief. The longest dash in Olympic competition is the 400 meter dash. Why isn't there a one mile dash? Because it's impossible to sprint for a whole mile. If it were workout volume which were responsible for large gains in muscular weight, you would see massively developed distance runners. Instead they have small, stringy muscles. Why? They are grossly overtrained!
The three principles of making large gains in muscular mass and strength, are: To be productive, training must be intense, brief, and infrequent.