If you mean the water feed valve then read on.
Place a piece of wood across the cistern (lid off of course) and tie up the float arm so water shuts off. Unscrew the ball float and shake to see if water has got in if so replace ball float see if drips stop.
IF there is no water in the ball float then replace it back on the float arm and turn off the water to the cistern inlet valve if plastic type remove the float arm unscrew the large nut on end of valve remove the plunger and washer, replace the washer.
Bob
January 2012