Before attempting to start your ATV, check for evidence of visiting or nesting rodents.
Check tire pressure to be sure they are properly inflated.
Check for cracks in the side walls of the tires.
Make sure all levers, pivot points and cables move freely without binding or excessive resistance.
Check the suspension for smooth travel.
Check the battery for full charge and recharge if necessary. If the battery was completely discharged you may want to consider replacing it. A battery that has sat discharged for a long period of time may have frozen and will never hold a full charge again.
Check all fluid levels. It is recommended that you change the oil before starting your ATV just in case condensation occurred over the winter.
Inspect rubber coolant hoses, fuel lines and CV boots for cracking or signs of wear.
Check conditions of the brakes and condition of the brake fluid.
Remove and inspect the spark plug and re-oil your cylinder before starting the engine.
Rotate the engine by using the pull cord to spread the oil on the cylinder walls.
Install the spark plug.
If you used a fuel stabilizer you are likely to be in good shape. If you drained the fuel system you will need to prime the system. Priming is usually as easy as turning the fuel switch to the on position.
Once you are certain that everything is working, try to start the engine. Do not hold the starter switch for more than a few seconds so you do not overheat the starter. Be sure to allow the starter to cool between attempts. After a few attempts the engine should start. If the engine does not seem to start after a few tries, double check all the proceeding steps.