I'm going to assume that you mean linking two computers together. If this
is the case, both PCs need to have a network card and you need a hub/switch
and 2 CAT5 patch cables. You install the network card (assuming the PCs do
not have built in network cards), connect the patch cables from the network
card to the hub/switch. This take care of the physical connection.
Now you have to deal with the OS configuration to get the two PCs to talk to
each other. Assuming this is XP (or an NT family OS) you will need to
configure the network cards on each PC to be on the name network. During
the installation of the network cards you'll be asked to set IP addresses
dynamically or manual. For sake of argument choose manual and enter IP
addresses. Set one to 192.168.0.1 and the other to 192.168.0.2 and set the
subnet mask on both to 255.255.255.0.
Now you have to enable sharing on the PCs. There is a setup home network
wizard in XP that you can run that will assist you with that.
HTH.