Hopefully you are reading this to educate yourself before disaster strikes. Regardless, this post will be short and to the point. Here's how to turn your home's water off in case of an emergency.
FIND YOUR WATER SHUT OFF VALVE
Most (but not all) houses have a main shutoff valve next to the house. It can be in the front yard, side yard, or even inside the garage. There are 2 types of water valves you can encounter:
TO SHUT OFF: Turn the valve clockwise.
*These valves have a propensity for failing. The handle might just spin endlessly, or close but not fully stop your water.
After closing the valve, open your faucets inside and see if water eventually (within a minute or two) stops coming out.
TO SHUT OFF: Turn the handle 90°, so it is perpendicular to the pipe.
This style does not often fail, but to make sure works and that it is the correct valve, open your faucets inside and confirm that water eventually (within a minute or two) stops coming out.
This is your gas meter. Don't stress out trying to figure out how this relates to your water system. It's not part of your water system. Keep looking for a valve like one listed to the left.
IF THAT DOESN'T WORK
As a last resort, you can try to turn the water off at the meter.
Find your water meter. In most cases the water meter will be at the front of your property next to the street, although in some cases it is elsewhere, such as the back edge of your property. If you just can't find it, a call to the water company could determine its location.
Locate the shut-off valve. Inside the water meter vault, you will find a valve. Dirt tends to pile up in these vaults, so the valve could be buried. Dig a little if need be.
Turn off your water. A water meter key is the proper tool to to shut the street valve, although in a pinch you could use a pair of pliers.
Check your meter. To make sure the water has been turned off, look at the water meter and confirm that it isn't showing movement (either with a changing digital readout or with a spinning dial).
Get your leak fixed! Call a professional so you can put this trouble behind you.