Lawn mowers not starting may have many causes. Maybe the gas is old, the spark plug is dirty or the engine is flooded. Engines flood with gas because they won't start properly. They can even flood with water if not stored properly. Either way, a flooded engine needs to be drained immediately, because attempting to start a flooded engine can destroy the internal parts of the engine.
Attempt to start the engine. If the engine does not start, do not attempt to restart more than three times. If the engine has run in the last seven days, then it is most likely flooded with gas. Wait 10 minutes and attempt to start it again. If the mower has been stored, uncovered, outside for an extended period of time, then the engine could be flooded with water.
Locate the spark plug(s) on the engine. They will be located either on the front corners of the engine or on top of the engine. They can be found by following the large plug wires to the plug locations.
Remove the plug boot(s). These are removed by grasping the boot and wiggling while pulling until the boot "pops" off the plug.
Remove the spark plug(s) with the socket and ratchet.
Turn the engine over several times to clear the liquid from the cylinders. The liquid will "blow" out of the spark plug holes. When no liquid comes out of the holes, the cylinders are clear.
Reinstall the spark plugs in the holes. Do not over-tighten the plugs, as this will damage the plugs and threads.
Reinstall the boots on the plugs. Press the boots onto the plugs until they "click" in place.
Start the engine.