Here is a Terrific Information about the Importance of Knowing Where Your Shutoff Valves Are! Compliments of Janine Barrett’s article in Home Care Buzz.
The last thing you want to worry about when you need to shut off the water (or gas) is having to search for the main valve. Take some time to locate your utility shutoff valves as well as your main electricity panel, so you can act quickly when needed. Here is some guidance to help you.
Gas shut off valves Typically, you’ll find your home’s gas shutoff valve near your gas meter. You will need a large crescent wrench to turn off the gas. Once the gas is off, do not try to turn it back on yourself. Call your utility company or another qualified professional. If you smell gas or suspect a leak, leave the area immediately and call your utility company
For some homes, the main circuit panel or electrical box is outside, where the power lines connect to the home. In newer homes, the service box is often in the garage. When you experience a power loss, open the panel and see if all of the breakers are lined up. If they are, the power loss is not because of a tripped circuit. If you see a circuit breaker out of alignment, turn it off, then back on. (It should be aligned with the rest of the breakers.)
The main water shutoff valve for your house looks like a large, metal valve with a sturdy handle. Usually, you’ll find an outside water shutoff valve below the hose bibb in the front of your house. Sometimes, it will be in the enclosure that houses an outside water heater. If the main shutoff is inside the house, you’ll find it near the water meter. Alternatively, it may be near the top of the water heater.
In most homes, each plumbing fixture has a shutoff valve. Be sure to find the shutoff valves for your kitchen sink, dishwasher, bathroom sinks, toilets, and washing machine. In the event of a leak, you will know how to shut off the water immediately.
The main shutoff valve for most irrigation systems is between a home’s main water supply shutoff and the irrigation controller. Typically, builders or landscapers install them in the basement or garage. If the main shutoff is outside, it will be near a faucet bibb. You may need adjustable pliers.
Source: Do You Know Where to Find Your Home’s Shutoff Valves?