The easiest way I learned Linux back on Slack 2.0 was to go to (which, back then wasn't much),
The Linux Documentation Project and try to learn about certain things, installing certain programs and actually turning an old box I had at the time into server serving FTP, HTTP, SMTP, POP3 & IMAP4.
You won't learn it in a short time, but do what interests you and you'll learn quicker than most. There is even a DOS->Linux Primer at that site if you're interested, but, if you're pretty good with OS X, then I trust you've seen the command line.