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.).
Best Answer
Unfortunately nothing in life is incredibly easy, and solar is no different. I would NOT suggest attempting to install it by yourself or even with the help of some untrained friends. Most average people wouldn't attempt to install their own electrical, or even their own water heater, but for whatever reason, some people believe that solar is easy. It's not. If you can't install your own water heater, you shouldn't even think about installing solar yourself.
What you can do, however, is analyze whether or not you are a good candidate for solar in the first place. You might find that you are a great candidate for solar, or you might be able to mark it off the list, narrowing down your options, making your decision eaier.
Question #1: Since you are in the Southern hemisphere, you want it on the North side of your roof. How much available roof space do you have on the North side? Solar hot water panels, for typical homes, will require an area between 9'x9' and 18'x9' in size (horizontal x vertical.) (Note: If there is no room on the Northern roof, but you can install them on the ground near your house, then that is also acceptable.)
Question #2: Are there any trees or other objects that will shade the Northern side of your roof (or the ground location)? If so, would it be reasonable to remove the objects causing the shade? Shade between 9:00 to 3:30 should be avoided, and shade between 10:00 and 2:30 is unacceptable.
Question #3: Is your storage acceptable for solar? You should have roughly 10 gal + 20 gal/person in storage. You should have an electric/gas heating element within the last 40 gallons of the hot water outlet... and plenty of storage near the cold inlet that is heated exclusively by solar. The element near the hot out is there to heat the water in case there is no solar input, and the storage space heated exclusively by solar is required so that the solar can have a chance to heat the water without having to compete with the electrical/gas element. For this reason, a two-tank setup is ideal. In the diagram below, the cold water enters the tank on the left, which should not be plugged in or connected to gas, instead being heated exclusively by solar, which feeds the tank on the right, which should be heated by gas/electric. With this setup, solar should be able to provide ~85% of your hot water.
Question #4: How many "sun-hours" do you receive in your location? (This is not equal to simply how many hours of the day that the Sun is out, but is a measure of how many watts per square meter fall at your location. It is a product of intensity/angle, duration, and cloud cover. Any measurement that you might find from a location within 100-150 km is sufficiently close.) Low exposure to the Sun can be compensated for by installing an additional panel, but if your area gets under 2.8 sunhours/day, I don't know that I'd recommend solar.
If any of the above concerns are applicable and not rectifiable, then you can cross solar hot water off the list. Unfortunately, not everyone is a good candidate for solar, and solar HW systems that are designed poorly are a waste of money, but well-designed systems installed on homes that meet these minimum requirements work exceptionally well and will produce hot water at a fraction of 15 cents/KWh electric.. or even natural gas. Though certain components will need replacing, sturdy flat-plate collectors should last 50-150 years (in a drainback system) and withstand up to baseball-sized hail.
Though I cannot install it for you, very detailed information is available on my web site's page on how solar hot water works.