Buy a wireless router. Buy an Xbox 360 Wireless Adaptor if you have the old xbox, but dont if have the slim one. Go to "Join Xbox Live" in your Xbox 360 Guide (Press the xbox logo on your controller). Follow the on-screen instructions.
Microsoft is trying to get away from the old xbox, because more people are buying the 360. What's the point of releasing an original xbox game if a small amount of people have the console. I suggest you buy the 360.