The first step to fixing a low pressure problem in a shower is identifying the cause. There are several, including debris in the water or other build ups that block the flow, a faulty pressure regulator or leak. Low water pressure in a shower can also be caused by low water pressure being supplied to the house in general.
Each possible reason for low pressure requires its own steps to resolve the issue.Debris
To check for debris in the pipes, remove the shower head and turn on the shower faucet. The water should come out in a strong flow. If it does, it is worthwhile checking the shower head, as that can become clogged with debris, too.Pressure regulator
Pressure regulators can become faulty. They control the water pressure that gets into the house, and if faulty should be repaired by a plumber.
Water leaks can also be the cause of a low pressure shower. Most fixes will have to be carried out by a plumber, but it is possible to check for leaks. One way is to shut off the water valve, which should stop the water. Check the water meter, and then leave it for a few hours. Come back and check again. If the reading has changed, then the water is still flowing, so there may be a leak.