And it's probably too much information to post in these threads.
But in a nutshell, these are your concerns:
Server: PDC (Primary Domain Controller) Windows 2000 Server operating system. Used for file and print sharing. Have your operating system on one drive and the shared files on a second drive. Add users and computers at the server. Read up on Active Directory.
Server BDC (Backup domain Controller) Not absolutely necessary, but absolutely necessary - if you know what I mean. Backup is the key word here.
20 Computers (clients) I prefer Windows 2000 Professional over Windows XP, but they'll be varying opinions in this regard. Nonetheless, they log into the Domain (log into the server) where group policies, passwords, etc. are established. More reading (for you) required here.
Internet access - Don't use the server for an Internet server. Buy a Router - a firewall/vpn router. Linksys has one (BEFVP41) for around $175. It's very user friendly, good documentation and tutorials can be found on-line, decent customer support, etc. The router plugs into the switch (or hub) just like the rest of the computers. Direct the computers to the router internal IP address as the Internet gateway.
Caution: Both the router and the server will be setup as a DHCP (Dynamic Host Configuration Protocol) server to automatically assign IP addresses to the network clients. Only one DHCP server is allowed per network, so one has to be disabled. I disable the router's DHCP. (In a small network, you may want to manually assign IP addresses, but it's almost a matter of personal preference.)
That's a start.