Official British Orienteering guide for newcomers here.
Beginners guide to Orienteering by Oli Johnson on the Planet Fear website. This guide is particularly suited to those who already run, or climb, or otherwise enjoy keeping fit in the outdoors but are yet to try orienteering.
Mole Valley Orienteers - Jargon buster If you're new, or just started, and baffled by all the jargon (there's lots of it!) this Jargon Buster should help you out.