The best way to learn is to dive right in and start playing around. OSX is UNIX so you can open the Terminal app and start playing around. Start with simple things like moving around the filesystem, copying/moving files and what. Googling "unix tutorial" and the like will probably be enough to get you started. Maybe play around with bash scripting. Try out utilities like grep, sed, awk, find. If you're into programming then maybe gcc, gdb, make.
The best way to get really good with UNIX is to move beyond your home directory. I learned by running my own servers. Things like setting up a web/ftp/mail/dns server and what not. Do it all on the command line. If you want to start doing this sort of thing, I'd definitely do it in a virtual machine so you don't run the risk of screwing up your OSX install. Download VirtualBox (free) and pick a *nix OS and go nuts. Linux is a popular choice. I'd personally go with FreeBSD since parts of OSX come from it (and I personally prefer it for servers). For example, lots of the user-land utilities are from FreeBSD (although they are similar on Linux). The FreeBSD handbook has a ton of info in it. Maybe try learning how to build and install a new kernel.
When you're on the command line, one of your most important tools is "man". It's the help system for *nix systems. Type "man <name of command>" and you'll see the "man page" (hit 'q' to exit). You'll also need to use a text editor a lot. If you want to go hard core, then use vi which is my editor of choice. It's available on every *nix OS and also has a steep learning curve.