First, you need to find your water shutoff valve. This is usually in your front yard near a hose bib, in your garage, or in line with your water softener unit. Shut off the valve, cutting off all water to your home.
Go in the house and turn on a faucet to make sure the water is off.
Check the flow-indicator triangle at the water meter. If the flow-indicator is moving, the leak is between the shutoff valve and the water meter on your property.
If you are not able to locate the leak, you may want to contact a professional plumber to locate and fix the leak.