A shower faucet's handle becomes hard to turn when either the valve stem's rubber washers have worn out and begin sticking to the water pipe's walls, or the splines on the handle or valve stem are damaged. The splines are small teeth located on the end of the valve stem and the inside of the faucet handle, which interlock with each other, causing the valve stem to turn when the handle turns so that the faucet turns on and off.
Turn off the water flow to the shower faucet by closing the plumbing main valve for your house. Turn on another faucet in the house to check that the water supply has been cut off completely before continuing.
Locate and remove the set screw holding the faucet's handle in place. The screw may be located under a cap in the middle of the handle, or it may be in the side of the handle near its base. Depending on the type of screw, you may need a Phillips screwdriver or Allen wrench to remove it.
Pull the handle off the faucet and examine the splines or small teeth inside the back of the handle, looking for damaged or missing teeth. Examine the splines on the valve stem where the handle sat, also looking for damaged or missing teeth. Take the old handle with you to a plumbing supply store if you find the handle's splines are damaged to find an exact match for a replacement handle.
Remove any nut on the valve stem with an adjustable wrench and unscrew the metal sleeve covering the rest of the valve stem or cartridge assembly. Pull out the metal pin or clip from the valve stem or cartridge, if one is present, using needle-nose pliers and turn the valve stem or cartridge counterclockwise to remove it from the water pipe.
Remove the screw from the end of the valve stem assembly and slide off the different parts. Coat the replacement washers with plumber's grease before putting the packing nut onto the valve stem first, then the packing washers, bonnet, bonnet washer, stem, seat washer and screw back on the valve stem assembly in that order. If you are installing a replacement valve cartridge, coat the rubber washers on the outside of the cartridge with plumber's grease before inserting the cartridge into the water pipe, with the notches in the cartridge lining up with the notches in the pipe.
Assemble the faucet again by reversing the removal steps you took previously. Once you have put the faucet back together, including replacing the handle and its retaining screw, restore the water to the faucet and the rest of the plumbing fixtures in your house by opening the main water valve.