How to setup server to share files?

If you want to share files locally, on your LAN, all you need is a switch or hub. Plug all your PCs into it, enable file sharing, and everyone should be able to see everyone else. A switch can be faster in some circumstances, but unless you are expecting the server to get pounded with data sharing requests, you can just as easily go with a hub.

If your file sharing needs are modest, then you probably don't need a server at all. You can share any file, folder, or printer on any of your PCs and the others will be able to see it and use it. This is peer-to-peer sharing, and you may want to start with this and move up to a dedicated server if you needs demand it.

If you have a broadband internet connection, you should have a router, period. A router will provide basic firewall protection against intrusions from the internet. In my opinion, every broadband connection should have a router installed for this reason alone.

You don't say so, but I assume you also want to share your cable internet connection. A router will help you here! All you should need to do is connect the hub or switch from my first paragraph into your router. After you reboot your PCs they should be able to get out on that internet connection.