Follow a good, structured and well balanced weight-lifting routine. Core basic moves are good for beginners: chins / pulldowns, military press, bench press, squat, deadlift / stiff-leg-deadlift.

Ensure you're eating sufficient protein and carbs - especially around workout times.

Not sure that getting big and muscular is such a great idea from a boxing perspective, though. But then I'm no stranger to the weight room, and something of a newbie to boxing.