You can think of the motion your arm makes when throwing the ball as a circular motion. If you're throwing a short distance, the circular motion will be smaller then when you are throwing farther, but it's still a circular motion. The circular motion will aid your throw by providing more natural momentum than simply bringing your arm straight back and then forward. The circular motion should begin when you're pulling the ball from your glove. If you are playing outfield you will almost always be making a longer throw, so when you remove the ball from your glove, your arm and hand should drop down and by your back knee. This will provide you with the longest circular motion possible. If you are making a shorter throw in the infield for example, you may take the ball out of your glove and move it back and down slightly. This will give you a circular motion appropriate for the distance.
It's important to have your hand on top of the ball as you pull it back and start your throwing motion.
How do you determine if you're throwing with a circular motion or not? One of the best ways to check yourself is to freeze occasionally after you pull the ball out of your glove. If you are bring it up and back for anything other than a very short throw, you are not using a good circular motion in your throw.
If you have been throwing incorrectly for a long time, then it is going to feel different throwing with a good circular motion. That is to be expected. Practice throwing this way all the time and it will soon feel natural and you should see increased accuracy and velocity.
When throwing you want your front shoulder to point in the direction of where you are throwing. So after fielding the ball you will be turning your body sideways and pointing your lead shoulder in the direction of the throw.
If you follow the logic of having your front shoulder facing the target then you might have guessed that you also want your lower body lined up in the same manner. Your back foot should be perpendicular to the target and your hips should be closed and also pointing in the direction of the target.
Once you have everything lined up, you'll want to step toward the target with your lead foot, push off your back leg, and throw the ball using your entire body.
In order to throw the ball so it won't tail, you want to make sure you throw it across all four seems with '12-6' rotation. '12-6' rotation refers to a clock. If the ball rotates from 12, straight down to where 6 would be on the clock, this would be considered '12-6' rotation. The next two images show an example of 12-6 rotation.
Unless you throw the ball straight over your head, you won't be able to get '12-6' rotation without moving your wrist. As the ball comes forward during your motion, you will want twist your wrist to keep your hand as vertical as possible. This is the key to having good '12-6' rotation on the ball.