Remove any grit or dust from the data side of your CD by blowing gently across the surface, then hold the CD under warm running water. Squirt some dish detergent onto the surface of the CD and gently wash it with your fingertip, moving from the inside to the outside in a straight line. Rinse it off and allow it to dry completely before trying to play it in your stereo again.
Wash the CD first, then squirt some liquid car wax onto the surface of the data side of the CD and work it in with a soft cloth, making sure to move in a straight line from the center of the CD to the outer edge. Let it dry, then buff out the wax with a clean cloth before attempting to play the CD again.
Squirt some baking soda toothpaste onto the data side of your CD, then work it firmly into the surface of the CD with a soft cloth, making sure to move in a straight line from the center to the outer edge. Polish the surface for several minutes, adding more toothpaste as needed, then rinse completely under warm water and allow to dry before trying to play the CD again.