Enroll in a class that educates you on how to identify wild mushrooms. Many botanical gardens, universities, biological stations and mushroom clubs offer classes on identifying wild mushrooms.
Read books and field guides on the different aspects of mushroom identification. Research the appearance, habitat they grow in and spore characteristics of each type. Many of the books have diagrams and photographs of wild mushrooms.
Inspect closely any wild mushrooms your instructor has available in class. Experience with the real fungi is invaluable in honing your identification skills. Keep in mind that even experienced gatherers sometimes ingest poisonous mushrooms, therefore it is wise to weigh the risks with the benefits before attempting to learn about this hobby.