Blaz is right.
Change the oil immediatly after the last mow ( a bit late now ) thus the mower sits with new clean fuel in the engine over winter.
Add some anti watering agent like Seafoam or Stabil into the remaining fuel in the tank, or better still drain the tank, put the fuel into your car & start with fresh fuel next season.
Clean the mower, particularly the underside and spray liberally with your favourite goo from a trigger pack , WD 40, RP 7, Armourall , etc etc etc.
The battery needs to be used, particularly is it is a Pb-Ca battery.
A good trick is to wire he mower battery into your car as in winter the loads on your battery icreases