I need to know the best and healthiest way to bulk up!?

ok so im 21 5'8" and 300 lbs, mainly fat, but with quite a bit of muscle too, i was recently offered a job as a club bouncer and i wanted to know what would be the best way to help pack on some muscle mainly in my chest, shoulders and arms, cuz thats where im least packed. ive tried working out my upper body vigorously, but to no avail. if someone knows how i can achieve my goal, please answer!!