It doesn't matter that you've neglected your lawn mower well into the season. Start caring for it now! First, install fresh spark plugs. They're inexpensive enough to replace rather than clean or gap.
If your mower has a paper air filter, give that a complete replacement, too. For foam air filters, buy new mower replacement foam and soak it oil before installing.
Dull blades harm lawns. Ripped out chunks of grass are highly vulnerable to a myriad of lawn diseases. Either remove the blade with a socket wrench, hone it with a file (following the existing cutting angle), or take it to a lawn-care shop for professional sharpening.
It's just good sense before doing any of this work to run the mower until it runs completely out of gas. Turn the mower filter-side up (to prevent clogging) and drain the oil. Be sure to remove the plug or plug wire to keep the mower from firing up while you're up to your elbows in machinery.
Be sure the tires are fully inflated – especially with riding mowers. Under-inflated tires on a riding mower can cause what we, in our family, usually refer to as crop circles – unevenly mowed patches that resemble that otherworld phenomenon that some people think exist.