I'm assuming you have highspeed internet?
If you do then you have a modem. From that modem you likely have either
1.) an ethernet cable running directly to your computer
2.) an ethernet cable running from modem to router to computer
3.) or an ethernet cable running from the modem to a wireless router that wirelessly connects to your computer.
If you have the first 2 setups then you just run an ethernet cord to the back of your 360 and that's that. But if you have the 3rd setup then you need the 100 wireless adapter (go to ebay for it)