Several vendors make this stuff...take a look at the link below.
Networking kits
I would go with D-Link or Linksys. Both are reasonably cheap and will get you started. Get PCI network interface cards (NIC) and not ISA.
To get started:
1. install the NIC cards on both machines.
2. Assuming your OS's are windoze 9x/NT, you need to set up Microsoft Networking
3. You will need to assign IP addresses (different for each machine), subnet mask (same for both), workgroup setting (same for both).
4. If you want to share files, you will need to add file sharing as well.
good luck!