My John Deere Riding mower has been sitting all winter. Now it won't start. Help?

Most likely a dumb question does it have gas in it ? Is it getting a spark to the plug ? How was it stored for winter was gas left in it all winter? back to you. AC

Pull the plug(s) and clean them.

While the plugs are out, put about half a teaspoon of oil down into each of the holes where the plugs go. Also put a small amount of fuel down there too.

Replace the plugs and then just keep cranking it over until she fires proper.

Edit:
I guess I should post a source for those individuals who aren't mechanics by trade...

Step 2 at Do It Yourself (http://www.diyornot.com/sample/diysteps.asp?ndx1=2&ndx2=7&ndx3=0&Rcd=235)

Why in the world would you want to put oil in the plug hole. There is only one reason to do that and that's for part of a compression test.

You must check for fuel, spark and air.

Spark = Yes/No? Replace and gap plug.
Air - Temporarily remove air cleaner
then...
Fuel - A little harder. But the first step is to remove plug. Place some carb cleaner in the spark plug hole. Attempt to start. See if the engine sputters at least once.

Results will dictate next steps.

I agree with KISS, do not pour oil in your cylinder. I use Sea Foam as a carb cleaner and just about everything else on a mower, you can mix it in the oil, in the gas or dump it down the throat of the carb while running. Make sure you drain your old gas and start with fresh gas. Spray a small amount of cold weather starting mixture down the carb as you crank.

remove the carb bowl and inspect. looking for gum, trash, or water.