1. Install NIC card on each PC.
2. Buy a switch.
3. Install RJ 45 wires from each PC NIC card to each port on your switch.
4. Assign static IP for each PC. From 192.168.0.1 to 192.168.0.5
5. Now you have a LAN.
6. Depending on what you want to do, the rest can vary widely. Tell us what you want to do and we'll see if we can advise you further.