On a walk-behind or riding lawn mower, they're usually near the front of the engine. On zero-turns, look near the back of the engine on the sides. Disconnect the spark plug lead and clean around the spark plug to remove any debris.
Ideally, most automobile spark plugs use a thread size of about 14mm thread why those of lawn mower range between 10 to 12 mm. Because of these differences in thread sizes, they just won't fit into the same hole or the unit in which the plug is to be fit in.