Prior to starting a gas lawn mower, make certain there is sufficient gas in the tank. If the machine is new or hasn't been used for some time, ensure there is oil in the crankcase.
Often there is a prime bulb on the engine for forcing gas into the carburetor. If so, squeeze this bulb two to three times before making an attempt to start the engine. Avoid priming too much as this floods the engine and prevents it from starting.
Give the starter rope a quick pull, which should start the engine. If unsuccessful, pull the rope two to three more times. If it still fails to start, reprime the engine, and try again. Check that the spark plug is in good condition and connected.