There are many excellent guide books to help one identify mushrooms. Probably the most thorough and detailed of these is David Arora’s book, Mushrooms Demystified (Ten Speed Press, Berkeley CA; second edition 1986).
In order to identify a mushroom, one must make very detailed observations of the mushroom and its spores and then use dichotomous keys (found in the guide books) to narrow down the possibilities until you think that you know which species of mushroom you have. It also helps to look up photographs of the mushroom, once you think you have figured out what it is, to make sure that you got it right. Here are a couple of web sites that I have found very useful for this purpose:
http://www.mushroomexpert.com/ This is an excellent website run by Michael Kuo, with beautiful pictures, descriptions and keys to identifying many species of mushrooms.
http://mushroomobserver.org/: This is an interactive website where experts and amateurs post photos of mushrooms that they have found. The purpose of this website is to record observations about mushrooms, help people to identify mushrooms that they are not familiar with, and expand the community around the scientific exploration of mushrooms (mycology).