The best thing you can do for your torso are sit-ups. Nothing fancy, just those bent-knees-and-feet-on-the-ground curls where you use your abdominal muscles to bring your shoulders up off the floor. Don't interlace your fingers behind your neck and pull your head forward as you try to curl up - you're risking neck injury or pain, not to mention you're not working your abs. Some people will lift their shoulders and back from the floor and work against gravity, others will bring their body up to the point their elbows touch their knees, and still others will alternate curls between right elbow-left knee and left elbow-right knee. In any case, you can overlap your fingers behind your head but don't lock your fingers together. Lift with your stomach. Start with 20 to 30.
For your arms, go with pushups. Keep your hands under your shoulders, your back and rump straight, and support yourself on the palms of your hands and your toes. Don't jerk yourself up and flop back to the ground between pushups. Raise yourself up slowly, and lower yourself slowly until your chest just grazes the ground. KEEP YOUR BACK STRAIGHT. If a conventional pushup is too much, put your knees on the ground instead of your toes. Try for a couple of sets of 20, but keep your back straight more important than the number.
Add arm and leg lifts for those back muscles along your spine. Lie prone with your arms extended in front of your head. Lift your right arm and left leg as high from the ground as you can and hold them for a count of five. Slowly bring them to the ground, and then lift your left arm and right leg. Start with 10 to 15 lifts on each side.
For the rotator cuff muscles in your shoulders, fasten an elastic cord to a doorknob. Put your back to the door and bring your elbow to your side and your forearm parallel to the ground and straight out from your side. Grasp the elastic cord and slowly rotate your arm until your forearm (still parallel to the floor) is pointing straight ahead of you. Ease your arm back.
Hold the elastic at one hip, grasp the elastic with the other hand, and rotate your forearm (keeping it parallel to the ground) until it's out about 45 degrees to the side.
How many of each exercise should you do? As with any exercise, start out by discussing with your doctor what you want to do, what you're going to do, and your goals. With that in hand, work until your muscles are tired but not worried. At most, do them every other day. Three times a week works just fine.