There are many variations and techniques used for serving in tennis. This Instructable will allow beginners to get the basic mechanics down for serving.
Step 1: What You'll Need
The type of tennis racket used and the tension of it's strings can greatly affect a tennis player's serve. For now, we won't worry too much about that since we'll mainly be going through the motions. So any regular tennis racket will do. For tennis balls, I recommend the standard Wilson brand although pretty much any kind will be fine.
Step 2: The Grip
Be sure to hold the racket at base of the handle! This is important for being able to extend your reach when swinging the racket.
Step 3: Positioning
Make sure your front foot is close to the baseline, but not touching it! This is known as a foot fault if you do, and your serve will not count.
A good rule of thumb is to have your toes pointed towards where you want to hit the ball to, in this case it is the opposite side service box.
Step 4: The Toss
Start with the ball in your hand at your side and your feet in the positioning discussed in the previous step. Keep your arm straight and toss the ball straight up in front of you. You should release the ball at a point so that it would fall straight back down from its peak. If you toss the ball up and keep your arm outreached, the ball should fall right back into your hand (otherwise on to the court if you were to let it drop all the way).
A good rule of thumb that I use: line your arm up with the net post as you are tossing the ball up. See the last picture of this step for a better description.
Step 5: The Swing
Raise the racket up and behind your head. The head of the racket should be behind your head just before the ball reaches its peak. As the ball reaches its peak, swing up and forward to hit the ball. Your arm should be completely straight as you make contact with the ball.
To practice the swing, you can practice hitting the ball with your racket starting in the position behind your head. Imagine that you are trying to scratch the back of your head with the strings of the racket. That is about where you want your racket to be just before you swing to hit the ball. The last picture of this step shows a good position to practice hitting the ball from.
Once you have just the swing down, then practice raising your racket into position as you are simultaneously tossing the ball up.
Step 6: Following Through
You should almost feel like you're "falling" into the court after your swing. It is helpful to go forward after your swing so that you are ready to play the point after serving!