To "winterize" means to prepare the home for when it's unoccupied.
The first step is to locate the water shut-off valve, preferably one that is outside.
With the water turned off, it's now time to turn off the water heater. A gas water heater should be shut off at the gas valve. An electric water heater needs to be shut off at the breaker.
Now that the water heater is turned off, drain the water. Open the faucets at the fixtures, and then go outside and open the outside sillcocks. Gravity will drain the water out of the heater.