Hi !
Do you need any special kind of UNIX ? Or any unix ? You can find a lot of free unix-like operating systems... Linux, FreeBSD, NetBSD, OpenBSD and lot of others... I used Linux for a while, but now I'm using FreeBSD. In my opinion is the best UNIX for beginners and for advanced users also... and you can find a lot of documentation on it too
If you want to download it go to
http://www.freebsd.org/ and you'll see there a few links...
If you would like Linux, I'll recomand you
http://www.linuxiso.org/ where you'll see a list of linux distros and the link to the CD images...
Anyway, there are more other unix-like operating systems, all you have to do is to search for them.
About understanding unix... you can start with any unix you like... it doesn't matter... the basics are the same everywhere
. It will matter only later, after you'll have some experience.
I hope this answer is satisfying
Good luck !
Bye Bye !