The serve is the most difficult shot in tennis but it is also essential to play a game. The serve must put the opponent in difficulty and must guarantee an advantageous starting point. If you miss the serve or if the serve is weak you give a considerable advantage to the opponent.
Learning the service requires a lot of practice.
In this video some very useful concepts and exercises for learning service are explained.
Part 1
1) Throwing the ball: arm straight and relaxed wrist (relaxed writst)
2) Keep the ball resting on the middle and ring fingers
3) Practice tossing the ball and look up in the sky
4) Throw the ball with your arm straight and let it fall
Part 2
1) Bend your knees at the same time you toss the ball
Part 3 - Practice with the arm holding the racket
1) Use the continental handle
2) Hold the ball in the arm that holds the racket
3) Bend your knees and throw the ball by rotating your body (from cutting position to parallel to the bottom line)
Part 4 - Practice with the racket
- In this exercise you don't have to do the whole movement yet
- Holding the arm with the racket up, practice throwing the ball and hit it without worrying about letting it enter the service area of the opponent's court.
Part 5 - full movement
1) Keep the weight 90% on the front leg and the 10% on the back leg
2) Shift your weight to the back leg
3) Return your weight to your front leg and bend your knees
4) Throw the ball and hit it with the top of the racket
English 
Italian