To flush a water heater you will need a garden hose long enough to reach outside to drain the water that that is in the water heater tank. If you cannot get the hose to a point that is lower than the water heater to discharge the water you will also need a pump, to pump the water out of the water heater.
Turn the water heater off, on an electric water heater turn the circuit breaker off, on a gas water heater turn the gas valve to the pilot position. You do not want to have the water heater fire while the tank is drained down.
Hook up the garden hose or, pump and hose to the water heater drain valve and open the drain valve allowing water to flow out under pressure. This will help to clear and or the mineral buildup that may be blocking the drain valve as well as establish a siphon if the hose alone is being used to drain the water heater. Is using the hose alone make sure that you get it to a point that is as low as possible below the water heater to speed the siphoning. Make sure water is coming out and be careful the water will be hot and if your water heater is turned up high the temperature of the water may cause scalding.
Turn off the water valve to the water heater and open a hot tap to allow air to enter the water heater. If you are using a pump, turn on the pump at this point. Allow the water heater to drain until empty.
Once the water heater is empty close the hot tap, and if a pump was being used disconnect the pump and hook the garden hose directly to the drain valve. Open the valve to the water heater allowing the incoming water to blast against the deposits on the bottom of the water heater breaking them up and pushing them out through the drain under pressure. Continue this until the white mineral chunks are no longer coming out of the hose and the water coming out is clear.
Close the valve supplying water to the water heater, close the drain valve, and open the hot tap once again. Open the supply valve to the water heater allowing the water heater to refill while you disconnect the hose and clean up the mess. Once the water heater has refilled and water is coming out of the hot tap you can close the hot tap and turn the water heater back on by turning on the circuit breaker or, the gas valve back into the normal operating position.
At this point you have completed flushing the water heater and it will take up to an hour before the tank is fully heated again. Take time to lift the lever on the T&P valve to test the valve and make sure it works.