IMPORTANT: It’s very simple to turn off your gas, but the gas company must come out to turn it back on, so don’t actually turn your gas off unless it is an emergency!
In order to turn your gas off, use a wrench, a pair of pliers, or a gas shut off tool to rotate the valve. When the valve is in line with the pipe, gas is flowing. When the valve is perpendicular to the pipe, the gas is off.
So, if you look at this picture, my gas is still on. (Notice, my tool and valve are parallel to the gas line.) If I were to turn the tool to the right, though, the tool and valve would be become perpendicular to the gas line, and I would know that my gas was off.
It is a good idea to keep a special gas wrench near your gas meter. Many people attach a gas wrench directly to the gas meter, but anybody could use it to turn your gas off, so you may choose to keep a gas wrench near the meter instead of directly attached to it.
You can find gas shut off tools at your local home improvement store or at a store that sells preparedness items. They’re very inexpensive.