I'm guessing you don't have the key to join the wireless network. If you have an ethernet cord you can hook your laptop directly into one of the numbered ports into the back of your router. After it's hooked up open a web browser such as internet explorer or safari. In the address bar type http://192.168.1.1
and a password dialog should pop up and the default router username is admin and password is admin. If thosed don't work try either admin blank or blank admin. After that you should have a page with tabs at the top click on the one that says wireless, then there should be a sub-tab called security that has the wireless key there.
If the admin password doesn't work there is a small red button on the back of the router that you can press with a pen and hold down for 15 seconds to reset the router to factory defaults that means no wireless password and that the wireless netwok will go back to the default name "linksys" . Though if you choose this route you might look into putting a password on your wireless depending on how crowded your neighborhood is. If you need any more help drop me a line on this question.