The Curveball Grip shown here is one of the most commonly used grips among professional pitchers. However, it is important to experiment with different grips that may work for you.  This particular grip has your middle finger and thumb with the majority of your total hand pressure. Your index finger should have little to no pressure on the ball with your thumb on the seam underneath.

Just before releasing the curveball, concentrate on pulling down on the top seam with the pressure of your middle finger while pushing up with the thumb on the bottom seam. This creates a higher percentage of top-spin, tilt, and depth on the ball for more vertical movement. The vertical movement you create will keep the ball off the plane of the bat and will look more enticing to hitters because the break stays on the plate instead of breaking sideways and off the plate.

As with all off speed pitches, it may take some time to get comfortable with the grip that works best for you.  Whichever grip you decide on, make sure you do not tip the hitters off by doing something like wiggling your glove while you secure your grip every time you throw an off-speed pitch.  As you move up the ranks in the game, hitters will pay closer attention to you movements and body language so they can get a better idea of what pitch is coming next.