Log into the router, (usually http://192.168.1.1 or http://192.168.0.1) On most routers the router's password will be under admin, the wireless password will be under the wireless security section.
If its a Linksys router, then the default IP is 192.168.1.1 and default username is <blank> and password is admin. Once logged in, you can go to the "management" section of your router and change the password / username