To do it wirelessly, you fit wireless network cards. These are commonly standard items on Laptops and other portable devices but not desktop PCs.

You would also be wanting either a wireless router (eg. the item I pointed to above for around £10) or a wireless access point.

In my case, I use a switch and a wireless access point. I live in an L shaped bungalow and the phone line comes in at one extreme (which is my room) and I have a wired router there. The most convenient location for wireless in terms of best coverage where it is most likely to be used however is the living room which is the other leg of the L and the access point is located in that room.