1)Open the web browser that you use: Firefox,Google Chrome,IE,or Opera and type in the IP address for your router,and press the Enter key. http://www.routeripaddress.com/< Router ip addresses can be found here
2)A window will open and then type in your routers default username and password,and press the Enter key again http://www.routerpasswords.com/< Router usernames and passwords found here
3)Navigate to the Wireless Security\Network Security Settings Tab and open it to locate or change your WEP or WPA Security Passphrase. Use WPA because it provides better security.
4)Write this Security Passphrase down and put it in a safe place to access when needed.
If you remember how you set the wireless network up you will have to either go into the routers internal settings or in one of the computers currently connected to the network.
To get into the routers internal settings follow Sr-72 Blackbirds post and you will find your wireless security key if that's what your needing.
If you set up your network by following the Windows network set up then go to a computer that is currently connected to your network and open the control panel and select Network and Internet, then select Homegroup or network depending on what OS your using and on the next page select view or print the password. The next page should have your password.