You must hit the ball at the peak of the toss, when you hit your tennis serve make sure the ball is hitting at the top portion of the racquet. Where you hit the ball on your racquet will determine the speed and accuracy in your tennis serve technique. While hitting the ball make sure your eyes are watching the tennis ball hit the tennis racquet! This will prevent mis-hits or missing the ball completely (which can be embarrassing!), so make sure your eye is on that ball!!!
A great way to test if you are tossing the ball high enough is to stand next to a fence and toss the ball, hit the ball with your racquet at the height of the toss and pin in against the fence and make sure your racquet is catching it on the top part of the racquet.
Here are some more great tennis serve techniques in greater detail you must also master when perfecting your tennis serve!