Turn the gas valve to "Pilot" if your water heater runs off gas. If it is electric, turn off the circuit breaker designated for your water tank. This prevents the tank from coming on while you are servicing it.
Connect a garden hose to the drain valve located on the bottom of the water tank. If you did not allow time for the water in the tank to cool off, expect the water to come out extremely hot. Position the other end of the hose into a drain or bucket away from children and pets.
Turn off the water inlet valve that feeds water into the water heater.
Open the pressure relief valve located on top of the water heater. If the heater does not have a pressure relief valve, open a hot water valve on a sink for two minutes to drain some water and pressure out of the tank.
Open the valve at the bottom of the tank and allow all of the sediment in the tank to drain out. It is possible for the hose to clog. If this happens, close the pressure valve and turn on the water supply. This will build up pressure and clear the hose. If no water comes out of the hose, wait an hour for the water to cool off and then remove the garden hose. Remove the valve with a pipe wrench and stick a screwdriver into the hole in the bottom of the tank to break up the sediment. Reattach the valve and hose. Attempt to service the tank again.
Allow the water to flow from the tank until the water flows clean. Once clean, close the valve and remove the hose. Close the water pressure relief valve and turn the water back on. Do not forget to turn the gas valve back to "Heat" or to turn on the breaker.