Vacationing - water heater off?
If you're only worried about the water heater, then you can turn it off and drain it.
Shut off the gas
You should see a gas pipe connected to the unit. It will likely either be rigid black pipe, or flexible yellow pipe.  Follow that pipe until you find the shutoff valve. It should be a quarter-turn ball valve, so you'll only have to turn it a quarter turn to shut it off.
An easy way to tell if the valve is open or closed, is to look at the handle.  If the handle is in line with the pipe (parallel), then the valve is open. If the handle is across the pipe  (perpendicular), then the valve is closed.
Shut off the water
Since you're only concerned about the tank, you only have to turn off the water feeding the tank.  Locate the cold water supply pipe (possibly marked with a blue ring on the tank). Follow the pipe until you find a valve. In this case, it could be a ball valve or a gate valve.  As with the gas valve, a ball valve will only require a quarter turn. A gate valve, however, will require multiple turns to shut.
Drain the tank
Connect one end of a garden hose to the spigot at the bottom of the tank, and run the other end to a drain that's lower than the spigot.  Open the hot water valve on a few faucets in the house, to allow air into the plumbing.  Open the valve on the spigot at the bottom of the tank.
CAUTION The water draining from the tank will be HOT.
If you have time,  turn the tank off and allow the water to cool before draining the tank. This could take a bit, as the tank is designed to hold the heat.
When you get home
When you come back, make sure the spigot at the bottom of the tank is closed. Open the valve on the cold water supply line, and allow the tank to fill. You'll know it's full, when water starts coming out of the faucets you opened throughout the house.
NOTE: As the plumbing fills, air will come out of the open faucets. As the system nears full, water and air will spit and sputter from the faucets.
Once full, close the faucets.  Turn on the gas, and follow the instructions on the tank to turn it on (light pilot, etc.).