You will need strength and power in your legs, core and arm ( particularly a strong shoulder ). Power needs to be transferred from your legs through your core to your throwing arm. This will increase your arm speed and MPH. Many athletes have strong, powerful legs but weak cores.
Along with your weightlifting program, you need to include power exercises like squat jumps, sport-specific plyometric exercises and sport-specific medicine ball exercises. These power exercises should be performed at FULL SPEED.
Improve your throwing arm's MPH by "powerizing" your entire body.