if there's a problem with one of your appliances, first turn off the gas supply to your home. Look for where the gas pipe enters through the wall of your house. You should find a service valve clearly marked. Be aware, there could be more than one, so check for others.
You can also shut off the gas at your tank. Simply turn the valve wheel. If the tank lid is locked and/or you are out and someone else needs to do it, then the service valve mentioned before can be used.