If you mean the wireless security key, you can find out what it is by opening your router's configuration utility in a web browser and looking at the "Wireless settings" section.
Using a wired connection to the router, open a web browser, type the router's internal IP Number into the address bar, then press <Enter>.
Internal IP numbers vary by make of router, but these are the usual ones:
Netgear routers: 192.168.0.1
Linksys routers: 192.168.1.1
D-Link router: 192.168.0.1
If none of the above numbers work, look in your router's documentation.