The easiest way is to plug a cat5 network cable into the xbox theres a port on the back click here and run it too the asdl router. Is your modem rj-45 or usb? Is it a modem + router built into one? Or i think you can plug it into an network router also. And then you need an Xbox Live starter kit.
On the link there is a picture and it is the port on the right hand side. An ethernet RJ-45 cat 5 cable is all that is needed!!