You'll need to get the Xbox LIVE starter kit for one.
Second, you'll need a length of LAN cable.
Third, you're going to need to get a router. I'm assuming here that you are also going to use the Broadband connection for your PC, so to hook both the Xbox and the PC up to the broadband connection, you'll want a router.
Fourth, you'll need to talk to your cable company about the IP addressing. If your provider uses static IP's (you have the same IP address all the time) you're all set. If your provider used dynamic IP's (they change periodically) you'll need to talk to your broadband provider about getting a dedicated IP for your Xbox. LIVE doesn't like dynamic IP's.