One way would be courses and certifications of course, though my experience with them has been poor... Lots of canned problems and canned answers, sometimes some really distro-specific things not applicable anywhere else, and not a lot of help teaching you how to troubleshoot. You will need to learn the basics to make much sense of it of course; file permissions, users, ownership and groups, disks and partitions are radically different from Windows' organization.

Install a variety of UNIX or Linux on a computer of your own -- doesn't have to be on your 'good' computer, in fact, probably better it isn't in case something goes seriously wrong. Most any "throwaway" PIII/PIV with 512M of RAM or better is great for a home server. Linux technically isn't UNIX by the way -- in the strictest sense that means an OS literally descended from one of the original UNIX varieties, but Linux was made from scratch and distanced from UNIX for copyright reasons. FreeBSD, NetBSD, or OpenSolaris are open varieties of UNIX. Don't install one of the toy Linux varieties(Ubuntu, Knoppix, Mandriva, anything really graphically-oriented) -- the GUI pretty much takes over those and you won't learn a lot. Try Gentoo, or Debian, or Fedoracore.

And once you have it, seriously use it. Make a home webserver/fileserver, get SSH going for remote access, see what problems you have to fight through to make things work.