cant connect a router… ;0

i bought a linksys wireless-g broadband router model no=WRT54G for my computer tower. and i tryed to install it but the cable that plugs into the phoneline in the back of my computer dosent fit it is too big. i tryed to switch some of the wires to my dsl modem and it dident work so i tryed to plug it in to the internet conection in the back of my tower and it is not working there either can someone please please help me!!!