If you're not going to be riding your ATV in the Winter, you need to store it but like any machine, it must be stored properly to insure it will be in good working order in the Spring.
One of the first things you want to do before storing your ATV for the Winter is to wash it. Give it a good, solid cleaning.
Next take the time to change the oil and the filter. Remember, new oil will not go bad from sitting in an engine all winter, it's old oil that will cause corrosion and eat away at your engine, so best to get it out of there. A fresh oil change before Winter storage will neutralize the acids in the engine and allow the engine to rest peacefully until the Spring when you're ready to ride again.
After changing the oil, be sure to start your engine and let it run for a few minutes to run the new oil around inside the engine. Since you're only running your engine a few minutes, you wont contaminate the new oil.
If you plan on storing the ATV for the duration of the Winter, you will want to make sure that your carburetor has no fuel in it. If you don't drain the fuel tank of your ATV, at least turn off the fuel valve, this stops fuel from flowing into your carburetor. You will need to run the engine to get the fuel out of the carb but it only needs to run a few minutes. The engine will sag and will go low as the fuel runs out. If you don't drain the fuel before storage your carb will develop a varnish in the ports that is almost impossible to clean out.
Lastly, clean and lube everything that moves. If it is going to be in storage you want it well oiled so it won't dry out and freeze up during the winter storage.
These simple steps will insure you have a well running ATV when you're ready to jump back on in the Spring.