Mushrooms are the reproductive fruit of a fungus. They come from spores, and spores are everywhere. They like to grow where it is dark, damp, and fertile. The best way to get rid of them is to keep the grass short, eliminate leaves and other fallen vegetation and branches, clean up animal poop, and reduce shady areas. Pick the mushrooms as soon as you see them before they drop spores. If all else fails, spread a fungicide that you get at the same place you buy other plant and lawn products