need help badly setting up Xbox Live with a linksys router..

ive tried hundreds of sites trying to get help setting up my xbox live. when i first bought it the other day, i brought it to my friends house where i made my account and everything worked fine, we were able to play live for hours. Then i brought it home to my room, where i have a modem, and a wireless linksys router so my dad's laptop can have internet access. now i run an ethernet cable from a port in the router straight to the xbox, and i can never get it to work. i really am about to give up on this, which means i wasted 200 bucks. any help is appreciated.