Check the oil level and see if it smells like gas. If it does change the oil.
Remove spark plug and hold the end of plug wire approx 1/4" from ground and spin engine. Do you get a blue spark? If so you have good ignition system. Install new spark plug.
Remove carb from engine. Spray about 1 second worth of starting fluid into the intake. Try to start engine. Should fire and run for a few seconds. If it does you have a carb problem.