Well that's not an easy question to answer...depends on how you are.
For me, diving in and using linux was the impetus to learning about it.
A few tips:
First, when it comes to distros...don't post "Which distro is best for me", these have been done to death and just a quick search will give you all the answers you'd ever need....but don't rack your brain, pick one...any one (I'd suggest the big names first like Mandrake or Debian) and install it...try it...THEN if you run into specific problems...post them and details here. We'll help.
Second, to learn more...get interested...at first, ask yourself about something you want to do and search on how to do that one thing...from there, you'll learn a few things...then find out about another thing you want to do..etc.
Third..practice. Don't boot in windows unless you need to...this forces you to get comfortable...what boosted my knowledge (and I still consider myself newbie-ish in many ways) was when I kicked windows completely off my laptop...I was forcing myself to ONLY use linux.
After that...check out sites, read docs...sometimes at random...whatever interests you...I suggest The Linux Documentation Project at www.tldp.org
and this site for starters (read the forums..see if you can find answers and help other newbies...teaching them something usually helps you learn it yourself as well).
As a good starter reference book (if you want to spend the bucks) I recommend "Running Linux" from O'Reilly books.