mark,
On each computer that you want to share files, open Windows Explorer, navigate to the folder that you want to share (like My Documents), right click on the folder and select Sharing and Security. In the middle of the window click on "Share this folder on the network." You'll want to choose a name that's unique to that folder and the computer that it's on - for example, Mark's Documents (because if they're all called My Documents, things can get a little confusing). Windows Firewall will configure itself to share those folders and files within your network. You'll know if it's working if the folder icon changes to one that shows a little hand holding the folder now.
If all of your computers are sharing the same wireless router, and they all have access to the Internet, you shouldn't have to do anything else to your network equipment.