A good place to download Linux is from linuxiso.org. They also have a good forum for Linux issues.
A good site to find documentation about Linux is
http://www.tldp.org/ and to see a blurb about each distro visit
http://www.distrowatch.com/ Each distro will also have documentation on its website (well, most that I've looked for, which isn't very many).
Fedora (used to be called Redhat) and Mandrake were supposed to be the best distros for new users. But I've also heard good things about Knoppix, because like M O'Brien says you can run it from the CD. I haven't tried Knoppix myself. Definitely a good way to try out Linux without having to install anything.