concentrate on cardio and some boxing specific training. you dont need to bulk up and cut down for boxing and u dnt need to follow a bodybuilding program, you will build useless muscle that will slow you down. jus get in a boxing gym if the trainers know ur a beginner they will build you up slowly, or they should do, just train and eat clean and your body will find its natural weight, and you will add muscle gradually, and most of all ENJOY!!
hope that helps.