You need high intensity workouts, because long ones build your stamina, but the shorter intense one help your muscles.
Try doing alot of squats sprints, and core exercises.
The core will help your shots for more power, and so will the legs.
Do lunges alot, untill you burn significantly.
Do squats with weights.
Do alot of medicine ball workouts, grab a partner and throw the ball using tennis like motions, such as hitting a forehand but letting go off the ball and your partner catching it.
try overheads with medicine balls too.
If you have ever gone to a gyms cardio section some of them have a stair climber, these help alot if you put the resistance up.
With these exercises though you have to follow through and keep up with them, and work hard.
Make sure you feel the burn, because without it, it doesn't do alot.