Could be a number of things, but to answer your points ;-
Oil level - left hand side of engine at rear - there is a round plastic plug with 2 'prongs' sticking out of it. Unscrew it. Oil should be up to the top of the hole.
Spark plug - has thick wire attached at the front of the engine. Pull off wire. Unscrew plug using plug spanner. Replace with new plug once a year. (Possibly not necessary, but if you don;t know what a 'good' plug looks like, they're cheap enough to just replace)
Air filter - rectangular(ish) tin box secured to top right of engine with 1 bolt. Remove bolt, separate 2 halves of box and remove element. Clean it in warm soapy water and dry properly(or replace it). Soak in clean engne oil. Squeeze out excess and replace in tin box.
Before refitting filter to engine, look down into the carburettor throat (where the filter fits) and press the primer button. There should be an obvious jet of fuel squirted from the jet into the engine. If not, you need a new carb diaphragm (at least). If OKI, refit filter and test.
phil_saunders@bigfoot.com
Phil Saunders
September 2008
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.