Working with just body weight, I suggest dividing up your target regions into arms/chest, back, legs and core. Push ups, sit ups, planks(both center right and left), pull ups with hands facing away, chin ups, squats, lunges and shoulder tubing exercises. most importantly work your flexibility and stretch, stretch, stretch. That's a HUGE part of staying healthy and throwing harder. Be sure to stretch after your throw as well. Good luck!