Servers :: How To Set To Up File Server In Office

I was hoping if someone could give me some suggestions on this matter. I have some IT knowledge but a bit rusty.I currently have 25 pcs all running web based applications on their own. So filesharing is a headache. I decided to setup a small file server for the office.This is what I need the server to do.1) Internet connection comes through server protected by a firewall and then distributed to the 5 pcs (star topology)2) Print and file sharing.3) application sharing (accounting software)4) fax facilitiesMy question to you all:How do I go about setting this up. What do I need (OS, softwares, etc)How do I secure the internet and distribute it to the rest.witch, router, etc)