There should be a white plastic plug with a hex head next to the gas valve. Just remove the plug and drain the water. You may need to open the pressure relief valve to let air in to let the water drain faster. That is all you need to do to Winterize the water heater. You DO need to also drain all the water lines.
Here is a picture of a mod done by JohnB
to make it easier to drain the water heater. Follow the hose on the left and you will see where the plastic plug is located. ( with thanks to JohnB for letting me "steal" his picture