to connect wirelessly through your computer you have two options:-
1. buy the official M$ 802.11a/b/g adaptor (expensive)
2. buy a wireless gaming adaptor (Netgear / Linksys / Belkin) such as these :-http://www.ebuyer.com/cat/Wireless/subcat/...Gaming-Adapters
gaming bridges/adaptors will do the same job as the official
wifi adaptor. the only real downside is that they are a little bit
more difficult to set up and also need an extra power supply.
the best method would be to get hold of either a wired or wireless router.
use wired if your xbox360 is reasonably near to your modem / cable location.