The Serve:
To be successful in the modern game, players have to have an effective tennis serve. They have to be able to hit the ball hard but also with amazing accuracy. The more points you can take on your 
tennis serve with service winners and aces, the harder you will be to beat. Having a big serve also allows the server to utilise other weapons more often to maintain control of the point and finish a point quickly (serve +1)
Tip 1: a consistent ball toss in front of the body and a continuous, rhythmical swing
Tip 2: find a strong throwing position (+/- 90-100 degrees) prior to your forward swing
Tip 3: A fast arm action at the top of the swing is important to generate good exit speed on the ball
Tip 4: You have to be brave as well as confident to hit the big serve, particularly when there is score board pressure. Avoid thinking, “this is a big point”, instead think about where and how you’d like to hit the serve, then as NIKE suggests… JUST DO IT!