Yard Machine Riding Mower Won't Start?

From SF Gates:

1
Check the oil before you start a four-cycle mower that has been sitting for a long time. You may have neglected to replenish it the last time you used the mower.

2
Prime the engine by pushing the priming bulb three times. You should see fuel in the glass window behind the bulb. If your engine has a choke, turn it on. Try starting the engine. Turn the choke off after a few attempts and try again.
3
Give the spark plug wire a little tug to see if it's loose. If it is, push it onto the plug and try starting the mower. If there isn't any difference, remove the plug with a spark plug wrench and clean the gap with an emory board. If the plug looks worn, replace it with a new one.
4
Siphon the gas out of the tank with a siphon hose if changing or cleaning the plug has no effect. Store the gas in an approved container and take it to the local waste facility to dispose of it as hazardous waste. Pour new gas in the tank and reprime the engine. If the mower has a two-cycle engine, be sure to mix the gas with the proper amount of two-cycle oil.
5
Stop trying to start the engine whenever you notice the smell of gas becoming strong. That usually means the carburetor is flooded. Wait five minutes before trying again.