Ok, You are gonna need a router.
Here's a link for a wired router. I perfer DLink.
D-Link - 4-Port Ethernet Broadband Router - EBR-2310
If you have anything wireless or plan on using wireless then get a wireless router. You don't have to use it's wireless features just because it's wireless. You can use it just like an original wired router.
Here's a link of what a wireless router would look like.
D-Link - RangeBooster G Wireless Router - WBR-2310
Here is a tutorial thread I made awhile ago on how to setup xbc.
XBOX Connect Tutorial
If you are thinking, "wow these routers are way too expensive." Don't worry, ignore them online prices. Just go to your local best buy or circuit city and you should find something affordable.
If you are still confused just hit me up on skype or aim.