I think the best way to start to learn linux is to jump right in - Install a distro if you haven't already, and start playing around, edit some files, break some things, fire up a terminal and learn how to do things from the command line, break more things, try to fix them.
Once you get comfortable with stuff like that, you might want to look into building a Linux from scratch system (http://www.linuxfromscratch.org/
and maybe try Slackware out (link in my sig), I know switching to slackware I learned more about linux than I ever did on other distros.
^_^ of course, looking around online and reading books, guides, etc. is also a great idea, and how-to's and such as well, but I think the best way to learn is to experiment.