If the stain is your normal brownish color, then get a good grade of primer like Zinzer BIN and put a couple of heavy coats over the stain and feather out around the stain. Let each coat dry before the other. It may take three to four coats before you place your finish coat on. This depends on how dark and how brown the stain is. When the stain has completely gone, then put on your finish coat.
But, DO NOT PUT ON THE FINISH COAT UNTIL THE STAIN HAS DISAPPEARED.