I'm a beginner to Unix System Administration and I'd like to start first by having a server setup as a firewall. What would be a good distro to do this with? I'll be running it on an old computer. I plan on using command line only with this box.

Also, where can I start reading about doing this?

Currently, there will only be workstations under (is that the correct term?) this firewall.

Once I get my hands on more boxes though I plan on setting up some more servers.

You can install any standred Linux distribution or *BSD distribution. Linux distribution comes with iptables command-line to setup "netfilter" framed firewall. BSD and Solaris provides ipf command-line for setting-up their firewall subsystems!!!

One of the easiest (by far) to get setup is Mandriva as it comes with shorewall (an iptables helper). It makes setting up iptables a breeze. Use that with webmin (a really full featured & mature web based server admin tool) and 90% of your work is done from the start.

Mandriva installs both when you install the OS in most cases. Then just go to http://localhost:10000 in a web browser on the server to be your firewall.

Once in look at the Networking tab or menu (depending on the theme at the time). You will see the Shoreline (yes shoreline is the same as shorewall) option. Just select Shoreline and get familiar with that tool. There are many docs on the Net about using this tool as well.

Cheers