I have to say that I'm surprised how many on this thread lament the weakness of their serves. The 3 most important shots in tennis are the serve, return of serve and cross court groundstrokes, in that order. If you are committed to your game, those shots are what you should be spending your practice time on. Mastering those shots is what will pay the highest dividends in match play. My serve is my best shot and my biggest weapon. It is nice to know that if you serve well, it's almost impossible to be broken.
So, having said that, the serve in tennis is based on throwing. The pitching motion is the same as the service motion - you gently place (I don't like the word toss), the ball into a box in the air with one hand, and you throw the racquet at the ball with a pitching motion with the other hand. It's not much more complicated than that.
Here is one of the all time great pitchers (even if he is a dang lefty), Sandy Koufax. Notice that, in the windup, his shoulders are nearly vertical with his throwing shoulder below his other shoulder, and at the point of release, his shoulders are nearly vertical again, the shoulders having reversed positions with the throwing shoulder now above the opposite shoulder and in line with his arm. This is how he generates his amazing upper body rotation. UBR is the key to serves and groundstrokes. Swinging with the arm only from the shoulder is defective technique in the modern game. You have to maximize your UBR. Koufax's UBR and arm motion is what you should emulate in your service motion:
Here are two of the best serving lessons I've found online. Although neither explicitly explains that they are teaching UBR, that is exactly what they are teaching, using mataphors to explain it:
I hope you all find this helpful.