where is it asking for it, on the wireless device your are trying to connect up or is the router is asking for it in a setup?
If it is a wireless device asking for it, your need to directly connect to the router and enter the set in a web browser, in there it will tell you what the key is.
First you connect to teh router via wire (Ethernet) and than go the admin page (usually http:// 192.168.0.1 or 192.168.1.1). Log in and go the wireless setup page and select the security encoding and set corresponding key. Than configure your devices to use that key when connecting to your network.
Actually you should do a RTFM... it is all there...