Turn off the gas supply valve, typically located by your gas meter on the gas inlet pipe. Turn off the gas by rotating the valve one quarter turn with an adjustable wrench. When the gas is off, the valve’s oblong stem points perpendicular to the inlet pipe. Then call your gas utility or the fire department from a remote location. Do not return to your home until you know it is safe.
Central home air conditioner service systems consist of two major components: a condensing unit that sits outside your house, and the evaporator coil (often referred to as an A-coil) that sits in the plenum of your furnace or air handler. The refrigerant in the A-coil picks up the heat from your home and moves it to the outdoor condensing unit. The condensing unit fan blows outside air through the condensing coil to remove the heat. The condensing unit houses the three parts replaceable by a DIYer: the contactor, the start/run capacitor(s) and the condenser fan motor. The condensing unit also houses the compressor, but only a pro can replace that. The A-coil has no parts that can be serviced by a DIYer.