Generally, there are two ways to get UNIX or UNIX-like systems:
1. From a hardware vendor like IBM, Sun, Apple, or HP, already loaded on a computer you buy from them.
2. As a software package that you load yourself, generally on PCs of the sort that can run Windows. Nobody (well, almost nobody) uses genuine UNIX for this. The best choice is Linux, which is UNIX-like, but so close to it in how it behaves and how the commands work that if you pick up a general UNIX book it will mostly just apply to Linux. Of course, there are numerous Linux-specific books as well.
For Linux, you need to pick a distribution vendor. For beginners, the best choices are Mandrake or SuSE. Both available from places like CompUSA or Best Buy.
For more info on how to get started with Linux, read my article at
www.basepath.com/linux.