Basically, I've been weight lifting a couple times a week for fun for several years but never anything serious. When I go back to school in September, I'm going to be joining an MMA gym. Before then, I want to lose weight to drop into a weight class appropriate for my height (I could probably safely lose 30 pounds). At the same time, I want to build muscle before then. I'm not concerned too much with building any lean muscle, that'll come with training and I plan on doing some cardio stuff anyway.
I've heard that it is hard, if not impossible to build muscle while seriously burning fat, since we need excess calories to build muscle. How true is this? If I cannot lose weight and build muscle at the same time, which should I do first? And what would be the best way to maintain the muscle mass that I already have if I decide to just focus on losing weight?