It is very important to buy mushrooms from a good source like a grocer or supermarket. Never buy from an amateur who may or may not know to identify mushrooms correctly. It is also important to clean mushrooms thoroughly. The best way to clean mushrooms is a damp kitchen cloth or paper towel.  Wipe them well and remove any dirt that may be stuck to them. Never soak mushrooms in water, they are like a sponge and can absorb water. If the mushrooms are really dirty you can rinse them in cold water just long enough to clean them.