Pitching in fast-pitch softball is perhaps the most important factor in winning and losing games. A good softball pitcher can keep her team in any game even if the other team has better players. One of the keys to effective pitching is being able to throw a variety of pitches. The curveball is one of the most effective pitches in the game. Many softball hitters can attack a fastball, but lack the confidence and ability to adjust to the curveball. As a result, they stress out whenever they see a curveball pitcher. It's a great pitch to learn to throw.
Learn the strategies for curveballs. The pitcher can start a curveball so it looks like it is coming right over the plate, and then have it drop out of the strike zone just as it gets to the hitting zone. By that point, the hitter has committed to hitting the ball and misses it. The other type of curveball is one that looks like it is going to be out of the strike zone and then drops into it at the last instant. The result of this pitch is usually a called strike.
Grip the softball with your thumb running along the seams of the softball and your second and third fingers along the opposite seams. Do not squeeze the ball tightly with your fingers. In fact, you should have a light grip (about a "4" on a scale of 1-to-10; 10 being the tightest grip) with your index finger barely pressing down on the ball's seams.
Do not snap your wrist when you let go of the softball. When you throw a fastball, your wrist snaps to provide additional power. For a curveball, let the ball roll off your finger tips. The pressure comes from your thumb and middle finger, while the spin that comes off the ball will make it curve.
Do not change the speed of your arm, nor the angle of your pitching arm, when you are throwing the curve. The more you change those things, the greater the chance the batter has of realizing you are about to throw a curveball. You don't want to have any "tells" that allows the batter to know what is coming.
Learn the mechanics of the fastball before you try to learn to throw the curve. The fastball is the most important pitch in the game, and a curveball will not be as effective unless the pitcher already has a good fastball.