The very best way to get started is to join a mushroom club! The Northwest Mushroomers' Association has a great website with links to other area mushrooming clubs (http://www.northwestmushroomers.org/links.htm). Many organizations have organized forays, which is a great way to get acquainted with local mushroom "spots" and to learn ID and habitat tips from experienced mushroomers.
It helps immensely to have an experienced mushroomer along to help you on the first few forays, as even basic ID can be a tricky for some species and even safe, easy-to-ID mushrooms such as morels, oysters, and chanterelles have potentially poisonous semi-look-alikes that could fool a novice forager.
For ID, a good website is the mushroom matchmaker (http://forestry-dev.org/cgi-bin/matchmaker/MatchMaker.asp)-- this is a program where you can input characteristics of a mushroom you've found and it generates potential matches, though it's a bit tricky to use if you have never used mycology terminology. The "Bible" of mushrooming in the West is David Arora's "Mushrooms Demystified", a huge book that you probably don't want to lug around in the forest but that is a great source of info. David Arora's "All That the Rain Promises (And More)" is a good pocket guide, but it lacks a lot of common PNW mushrooms. "Mushrooms of the Pacific Northwest" by Steve Trudell and Joe Ammirati is a wonderful field guide (assuming you're in the Pacific Northwest)- while it doesn't have a great key it has great photos and descriptions of the majority of mushrooms you'll find around Western WA, at least.
As for where to go, that depends on what you want to find. You can probably find some delicious edibles in your lawn, in your neighbor's mulch, or at a local park... But for other species a trip up to the mountains or into the coastal woods is in order. The spring and fall are the best times to mushroom, though some species can be found in the summer and a few "snowbank mushrooms" can be found in late winter... So if you go on some unsuccessful forays this summer just keep in mind that when the rain starts up and the temps drop in the fall, you'll have more mushrooms than you know what to do with!
So anyways, get some good books, join a mushrooming club or find a knowledgeable friend, and soon you'll be enjoying the almondy richness of Agaricus augustus, the chicken-like taste of Laetiporus conifericola, the apricot scent of Cantharellus spp., and so on. The PNW is such a great place to forage!!! Just please do your research first- while few mushrooms are deadly many can make you miserable, and it's just not worth the risk... When in doubt, throw it out!