Meant to say I'm*
Mike: There are two ways.
You can buy a USB Wi-Fi adapter. These are very easy to use; just plug it into a USB port and install the software. It's something like this: http://www.amazon.com/Panda-300Mbps-Wire…
The other way is to install a PCI adapter. This involves opening your computer and installing the Wi-Fi card into an available PCI slot. It's still quite easy but takes a bit longer. I would go with this since the antennas are better than the ones on a USB adapter. http://www.amazon.com/TP-LINK-TL-WDN4800…
Although, I really encourage you to use Ethernet if possible. Wi-Fi, although very convenient, isn't the best choice for gaming. If you have no other option, then this should work fairly well, though.
Wi-Fi can be easily added via a USB Wi-Fi dongle, or a little less easily by installing a Wi-Fi expansion card. "Negotiator" is quite correct, however, and a wired connection should be used for gaming — wireless provides only 40% of the advertised data rate, e.g., 802.11g provides a raw data rate of 54Mbits/sec, but only 22Mbits/sec data transfer, with the rest lost to overhead (handshaking, authentication, encryption, etc.).
Ethernet is way better and more reliable for gamers.
hook it up to a router