Rob Berkes suggestion of a live CD is a good one. Personally I like Ubuntu which you can download here. Once you are up and running go to the menu in the top left and type "terminal" which should bring up a Terminal icon to click on. Once you have the command line you are in good shape to start looking around, installing packages, and trying things out.
This page lists a number of common server configurations and how to set them up with Ubuntu. Try installing some server packages, like Apache:
$ sudo apt-get install apache2
Once that's in place, open up Firefox on the machine you are on and visit "http://localhost". Try a bunch of things out, and don't be afraid to screw up. You can't break the system with a Live CD. If things get screwy, you can just reboot, and you'll be back to square one.
As you start to feel more comfortable interacting with the command line and package manager, you can start looking into what it takes to set up a DNS server (it's not the easiest thing for a newbie, but don't get discouraged!). And finally check out Linux System Administration, which runs the gamut of technologies you should at least be familiar with the existence of.
Good luck, and when you run into problems, don't hesitate to ask about them here!