hi there. When i was in a similar position to you, i found this book invaluable:
Unix Unleashed from SAMS Publishing
There are
a couple of different versions, not sure if you get all the chapters in the one i linked to. Anyway, this book does not assume any one type of *ix system, and everything in it should be platform independent. Given that it starts at the basics and covers just about everything, it really is invaluable to a newcomer, in my opinion.
If you haven't got a *ix on your PC, i would recommend using fedora or ubuntu to start with, or if you don't mind a steeper learning curve, use slackware. Slackware has fewer distro-specific commands and so in my opinion, it reflects "unix" better than the other two, but slackware is quite frustrating for the uninitiate, there's the tradeoff. You could also try one of the *BSDs or, Sun has finally made a "free" version
of Solaris for PC if you want to be a bit exotic. You may not be aware but very few systems actually run "UNIX" as such these days, though it depends if you count Solaris, AIX, HP/UX and *BSD as Unix, since none of them actually are UNIX, but they can all trace their parentage back to it. Linux, on the other hand, is a from-scratch UNIX clone, which uses large amounts of software from
GNU, which is a project which intends to provide a suite of software which clones UNIX software, but which is available for anyone to use, modify and redistribute. There are volumes of reading you could do on this, if interested, i would say,
start here and i suppose i should mention
this by way of balance.
Also, you have a head start in using *ix if you have MacOSX since it is a very distant descendent of UNIX (although i am sure somebody will "correct" me by elaborating!). I haven't really tried MacOSX, but i would be interested to hear how your experiences compare between it and other *ix systems.