The circle change can be a very good pitch, but you don't want to use it too much. It catches the batter off guard since it's off speed, and (if thrown correctly) also tails slightly in the opposite direction of a curve ball. It tails in the same direction as a slider, though not as drastically.
To grip the circle change, start by making a circle with your thumb and index finger. Use your other three fingers to grip the ball, gripping it far back, close to your palm. Gripping the ball more tightly, in the back of your hand, helps to take speed off your pitch. The grip is shown below:
If thrown correctly, the ball will come off your middle finger with a slight side spin, which causes it to tail. The arm motion is very similar to that of the fastball, though because you're gripping the ball with your outside three fingers, the ball will come off your hand differently than when throwing a fastball. Also, to take more speed off the pitch, you should drag your back foot. This will slow down your motion enough to take speed off the pitch, but will not be noticable to the batter.
One possible arm motion is shown below:
Though you want your change up to be slower than your fastball, you should still follow through completely after releasing the ball. This will help to prevent injury, and will keep the rest of your windup more consistent, so that your windup is not drastically different for each pitch you throw.