Turn off the water supply to your bathroom.
Pry off the cap of the handle over the problematic valve. Remove the screw holding the handle in place and pull the handle off.
Grab the washer at the end of the valve with a pair of pliers and pull it off. Inspect it for signs of cracking or wear. Replace it if necessary.
Inspect the valve for signs of rust or excessive white material -- mineral deposits -- if it is sticking or stuck. Clean the valve with a mixture of 50 percent white vinegar and 50 percent water to remove the white substance.
Tape the ends of a wrench with plumber's tape. Grab the valve with the wrench and pull on it to remove it from the wall. Cover the valve with a cartridge puller, which comes with your replacement valve, if it is stuck. Wiggle the cartridge puller with your wrench till the valve comes free.
Inspect all the washers and components of the valve. Replace individual components if necessary.
Insert a new valve into the hole in the wall, aligning it properly with the water supply line. if cleaning or replacing individual parts did not do the trick. Reassemble the unit.