I think you probably mean router, not server. A server would be 'serving' up stuff to the clients, like web pages, ftp etc. What I think you want is a router, which allows multiple 'client' machines (on an internal lan) to connect through the router to the internet. Basically you need to enable forwarding on the router machine and set up masquerading with iptables.
This link should get you started, but there's hundreds of howtos around.
