A router would be the easiest way to share the connection. Also a reouter is a good firewall. If you don't want to connect any wireless devices you can get a decent router for around $40 (more with wireless). I usually recommend Linksys brand. They make reliable equiptment and are typicaly one of the easiest to set up. But just about any brand would work.

You would then need one Ethernet (network) cable for each PC. Long enough to reach from the router to the PC.