First you need to decide which speed you need. If you are just going to be using the internet and sharing files then 802.11b will be sufficient with an 11Mbp/s transfer rate. If however, you want to play games then you will need the faster transfer rate of 54Mbp/s provided by the 802.11g standard.
Whichever speed you choose, you will need a router and then some way of connecting the PC's to the network. You can use a wireless USB adapter for connecting the PC's to the network for ease or you can insert PCI cards. If you are connecting with a laptop, then you can use a PCMIA card of course.
All of the things you will need can be bought online from eBuyer - click here.
I prefer Linksys - here are the links for the specific items.
Router (11Mbp/s - 802.11b) - click here
Router (54Mbp/s - 802.11g) - click here
USB Adapter (11Mbp/s) - click here
PCI (54Mbp/s Card) - click here