I would highly recommend that folks new to GNU/Linux start with the Knoppix live cd. With Knoppix 3.9 there is just one iso image to burn:
(http://debian.tu-bs.de/knoppix/powerPC/) (or go to knoppix.org for the pc version: lots of mirrors)
and booting the image is as easy as inserting the disk into a cd/dvd device, rebooting and pressing 'return/enter' at the prompt. Instant, well almost, GNU/Linux system that uses free space on your hard disk and provides a nice 'sandbox' to learn what is what, see how things work and find information about programs (and commands) with 'info ' and 'man ' on the CLI.
And speaking of the CLI, spend some time with the BSD CLI on OSX. It will help you be more at ease with the GNU/Linux CLI as they are very similar; both can be the 'bash' shell but there are file system differences.
Don't be intimidated by GNU/Linux. Just relax, have some fun and learn at the same time.
A Mac User Tries Linux Part 1