In order of UNIXness, here are some downloadable operating systems....
1. Solaris - go to
www.sun.com
2. OpenBSD, NetBSD, FreeBSD, goto
www.openbsd.org,
www.netbsd.org,
www.freebsd.org
3. Linux,
www.debian.com, etc,etc etc,
I suggest you learn
(a) what it is,
(b) it's philosophies and history,
(c) how to install a version,
(d) then how to use a shell,
(e) how to use "man"
(f) how to configure the system
(g) how to write simple scripts
(h) how to install new software/applications