Mycology is the branch of biology concerned with fungi: their classification, structure, genetics, ecology and uses. Fungi are a kingdom of their own, more closely related to animals than to plants, and the kingdom takes in not only mushrooms but yeasts, moulds, rusts and the fungi that form lichens.
Estimates of the number of fungal species run into the millions, and only around 150,000 have been formally described. Britain alone has many thousands of recorded species, and amateurs still add new county and national records every year. Few sciences leave so much for a beginner to find.
Start by looking. Pick one habitat close to home, a park, a wood or even a lawn, and visit it regularly through the autumn. Photograph what you find from above, from the side and from underneath, note what it was growing on, and make a spore print. Those four observations are what identification keys ask for.
Then learn the microscope. Most fungi cannot be named to species from their appearance alone; spore size, shape and ornamentation, and the cells on the gill edge, are what separate them. A compound microscope reaching 1000x is the single biggest step an amateur can take, and the microscopy kit page sets out what a first bench needs.
The British Mycological Society, founded in 1896, is the national learned society. It runs UK Fungus Day each autumn and supports a network of local recording groups. The Association of British Fungus Groups links many of those groups and is aimed squarely at amateurs.
Joining a local fungus group is the fastest way to learn, because identification is taught best in person, on a foray, by someone who has seen the species before. Records you make can go into the Fungal Records Database of Britain and Ireland, which is how amateur observations become useful science.
The Field Studies Council runs fungus identification courses at centres across England and Wales, and many local groups run beginners' forays every autumn. The Royal Botanic Gardens, Kew holds one of the largest fungaria in the world. University-level mycology in the UK is usually studied within microbiology, plant science or ecology degrees rather than as a subject of its own.
Fungi of Temperate Europe by Thomas Læssøe and Jens H. Petersen is the most complete photographic guide for Britain and Europe. Mushrooms by Roger Phillips is the classic British reference. Mushrooms and Toadstools of Britain and Europe by Geoffrey Kibby is excellent on difficult groups. For the wider story, Entangled Life by Merlin Sheldrake is the book that brought a new generation to fungi.
Spore microscopy is where many people's interest in fungi becomes properly scientific. A print gives you colour; the microscope gives you size, shape, wall thickness and the germ pore, the characters keys actually turn on. Reference material of known identity lets you calibrate your eye and your instrument against something certain, which is what a catalogued spore library is for. The MycoWorld library exists to supply exactly that.
Never eat a wild mushroom you have not identified with certainty, ideally confirmed by someone experienced: several British species are deadly. Some species are legally protected and must not be picked, and on nature reserves picking anything is usually forbidden. Fungi containing psilocybin are controlled drugs in any form. Spores are lawful for microscopy, which is the only use MycoWorld supplies them for.