Leaks water in bottom of fridge

We have a whirlpool refrigerator, it was made in 2000. It leaks water into the bottom of the refriderator from the freezer. What would cause this? We completely unloaded it and defrosted the entire refrigerator but it is still leaking. Would a faulty fan in the freezer be the problem? If so how simple is the fix? Is it a do yourselfer or should a pro do the work?