Before draining it, make sure the propane and/or electric is turned off.
#1, if you have an atwood water heater, there is not normally an anode rod in it, so remove the plastic plug
#2 if you have a suburban water heater, at least once a yr, pull out the anode rod and see if it is 90% gone, if so get a new one and while it is out.
Be sure water has cooled off before opening the water heater drain, no need to get burned by hot water, turn off water heater and run water out of a faucet till coolish.
Turn off city water/water pump and run faucet until pressure is all relieved, trust me on this one, don’t ask how I know but the water drain plug come sout like a bullet if system is still pressurized and water goes spraying EVERYWHERE.