Find your main shutoff valve. A lot of main valves are located inside a manhole cover like this one. Some are inside a cover that is only +/-10"-12"
Take the lid off the cover, this is what you should see.
Look for the arrow (which is the actual ball valve) located on the outlet end of the valve.
This is what you are looking for.
Put your pipe wrench on the valve head and turn clockwise.
Keep turning 180°.
The arrow will point in the opposite direction from where it was when you started. And the two holes on the side will line up. You water is now shut off.
Before cutting or working on your pipes, remember to "drain" the system. Do this by opening your faucet valves around the house. This releases the pressure from the system so You can work on it.