If your John Deere riding tractor runs properly but the light system will not work then it is most likely due to a blown fuse. Check the fuse, if it is blown then replace it with the correct amp fuse.
If the fuse is good then check the battery connections to make sure they are clean and tight. Corroded battery cables or terminals should be cleaned thoroughly.
You can find the model and serial numbers on most models of John Deere riding tractor on the front left side of the frame.
NOTE: if you need a part for the small engine in your John Deere lawn tractor, there will be a separate model/serial number for the engine. Here are the small engines that are available in your John Deere tractor. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: