When you have a leak in a Valley shower valve there are a couple of possible repairs that will stop the leak. The first and least expensive is to replace the seats and springs inside the valve. If this does not repair the leak the next thing to do is to replace the cartridge. The seats and springs should always be replaced when changing the cartridge anyway, so doing that first is part of the process and may save you the cost of a new cartridge.
The first thing to do is to shut off the water to the house. Drain any water remaining in the pipes by turning on the faucet in the bathroom sink.