Many players have problems in ensuring that their tennis serve is solid and consistent. For example, you might have the tendency of serving into the net. This might mean that you are making contact with the ball when it is far in front of yourself. You can rectify this by tossing the ball less far forward and leaning in more just before you make your swing.
You should also ensure that you do not toss the ball much higher than your point of contact. Such actions will lead to a downward vector when it descends toward your racket. Hence, try aiming to toss the ball no more than 3 to 4 inches higher than your point of contact.
Additionally, you should put some topspin on the ball when you serve. This ensures that the shot clears the net with room to spare as compared to a hard flat serve that will barely clear the net.