|You connect with an ethernet/cat-5 cable to the router. Then login to the default admin page; after-which you simply find your way to the wifi setup section, and the password will be there (in the secirity section).|
Depending onthe make of your router depends the actual ip address for your router's admin page.
Typically it's one of these.
192.168.1 or 192.168.0.1
Although there other vaitations too. Make/model of your router would allow pholks here to guide you more specifically/accurately.