Assuming you have no software firewalls and are using Windows XP:
If all the computers are already connected to the router and able to access the internet, you are 90% of the way there.
Open the control panel, then double click on the network setup wizard. You'll have to do this on each computer. Follow the wizard. Give your workgroup a name and tell it to enable file and printer sharing. Make sure each computer has a unique name and description, so it's easy to tell what you're looking at later when you search your home network. Once the wizard is finished, tell it to just finish... you don't need to create network setup disks or anything.
You may have to restart the computers...but that's it. They will all have access to each other, and any folder or file or printer that you set up as shared will be visible to all.
Tom already described how to share a printer and hook up to it with the other machines.