Practice your serving pitch. For both types of serve, toss the ball to your hitting hand and aim for the lower part of the ball. Try hitting the ball not with your palm but with the horseshoe part of your hand for a more accurate serve. Make sure you have positioned yourself and aimed correctly or the ball will go haywire.
Experiment with force. What's too much? What's too little? Soon enough your muscles will remember what works and you'll be able to aim the ball like a bullet out of a gun.