www.linux.org for starters. then start checking distro's websites "debian, suse, redhat, fedora, slackware, ubuntu etc"
ubuntu is the easiest install in the linux world imo. suse enterprise is great, too. polishlinux.org is a good place to compare distros - distrowatch.com is another good resource.
best bet, and what most people do, is pick one that you think you might like, and install it. no better way to learn than to dive right in. be prepared to read. seriously.