Do you have wireless, or is your router cable only? Does it have basic network instructions with it (follow them!)
If your router is wireless and one of your PC's is near a phone socket (ADSL?) plug the router into the phone socket with the cable/filter supplied(?)You can then connect the PC to the router by ethernet cable.
Or you can connect it wirelessly by a PCI network adapter (plug in thing with an antenna) or a USB adapter. Do the same for the other PC.
IF your router is ethernet only, both PC's will have to be connected by ethernet cables.
The rest is down to windows and setting up home networks and file sharing etc.