I think the 
best exercise for the chest muscles are pushups. There are many variations of pushups too. You can start with an incline push up if you need to build muscle and can't do a push up. Then start on your knees until you can work up to a push up on your toes. Another variation is to do a move where you start in a push up position and then lower yourself down to your elbows and then go back up on your hands. It helps to strengthen muscles!