A shower mixer tap has a single valve cartridge that you control by turning the faucet's handle. The shower's tap starts to leak water when the cartridge's rubber seals wear out and stop maintaining a watertight seal inside the tap's assembly. You must take apart the tap and install an exact replacement of the cartridge to stop the leaking. Take the worn cartridge with you to the store when you buy a replacement.
Turn off the water to the shower's tap by closing the main water valve in the house. Turn on another faucet on a lower level of the house or at an outside faucet to drain water from the lines.
Remove the screw from the shower tap's handle, which may be located near the base of the handle or under a plastic cap in the middle of the handle's post. Pull the handle off the faucet.
Pull off the metal sleeve from the faucet and slide the metal washer off the cartridge stem. Use needle nose pliers to pull out the retaining clip or pin from the cartridge.
Pull out the cartridge from the faucet using a pair of pliers or a cartridge puller tool. Coat the replacement cartridge's seals with plumber's grease and insert it into the faucet until it fits snugly.
Reassemble the rest of the faucet in the reverse order you took it apart. Open the shutoff valve and turn the faucet on to test for leaks.