How To Add An Ethernet?

You need to buy a networking card that is compatible with your machine - you need to know what kind of slots it has - PCI or PCIe (or ISA if you have an old machine, without more detail we can't help further) etc. Most cards will have drivers for XP so you won't have software problems. The port that ethernet uses is commonly call an RJ45 port.

As Marcus points out in the comments, USB -> RJ45 adaptors are a cheap and easy option, here's an example. USB is not an efficient communication port however, and a USB -> Ethernet connection will cause more overhead than a dedicated card, so if you have old hardware or you want to guarantee optimal performance, USB is not the ideal solution.

You'll need to obtain what's called a Network Interface Card (network card for short).

Do you have other computers to connect it to? There should be a "switch" somewhere that you will need to run a cable from your new network card to.

Alternately, if your network is wireless, you can get a wireless network card for your PC.

Give us a bit more detail about what you're trying to do, please.