high protien low fat. skinless chicken breast and fish are your meats of choice, then eat a lot of raw veggies. Whole wheat for breads.
Just join the boxing gym, who cares if you're a fatty, that weight will drop off a lot quicker. eat clean and box, you'll drop that fat very quickly, and you'll be improving your boxing at the same time.
Dropping fat before joining a gym is like shaving your head before getting a hair cut.