I'd suggest learning Linux specifically at this point, as it is quickly gaining popularity in many areas. Additionally, as Linux is part of the "UNIX family", what you learn about Linux will translate pretty well to other *NIX operating systems.
A great way to start learning Linux without even having to install it on your computer is to get one of the "Live" Linux versions which runs soley off of a CD. Knoppix is probably the most popular distribution of this type, but there are many others.
Although these versions of Linux will run more slowly than a version which you install directly to hard drive, because they do not alter your hard drive at all, you don't have to worry about modifying your existing (probably Windows) system in any way. Of course, if you want to dive right in to the real thing, you can either make a partition for Linux on your current hard drive or add a second drive just for Linux. If you currently have Windows installed on the machine you want to put Linux on, the Windows installation should be detected during the Linux installation process, and Linux will automatically configure your system to give you a choice of loading either Win or Linux when you boot.
There are tons of online Linux resources, the
Linux Documentation Project being one of the most comprehensive. The site has a huge collection of online books, guides, HOW-TOs, etc. covering aspects of Linux from beginner-level to quite advanced. I'd also suggest buying one or two hard-copy books as "hands-on" references. O'Reilly Press publishes many excellent Linux books; "Running Linux" and "Linux in a Nutshell" are two good titles for beginners.
For help with specific Linux questions, there are obviously many online Linux support sites with friendly, helpful, and knowledgeable members. My two favorites are *cough**shameless plug**cough*
www.justlinux.com and
www.linuxquestions.org.