This is a brief, no-nonsense approach to taking that lawn mower out of storage and getting it ready to cut grass for a new season. One of the first things to do is check the spark plug. Most lawn mowers only have one spark plug and if it is old, fouled or burned down, the mower will not work as efficiently as it should. It may be a good idea to replace the plug every year as it is an inexpensive item that can be found at most big box or auto parts stores.
Changing the oil in your lawn mower is a good idea also, but make sure you use oil design fro a lawn mower and not automobile oil. A trip to a store like Home Depot will allow you to select the proper oil type.Lawn mowers kick up a lot of dirt and grass clippings and that can clog up the air filter. If your lawn mower engine does not get enough air into the engine it could be hard to start and will not run as well as it should.
Take a look at the blade and underside of your mower to see if the blade needs to be sharpened and clean any excess grass that may have collected and hardened on the side of the body where the blade turns. If the blade is dull then it would be good to sharpen it to make sure you get even grass cutting.
If you can't perform all of these tasks yourself then take the mower to an authorized service center. Happy grass cutting.