Forgotten security key number

If by that you mean the router's WEP or WPA wireless security passphrase, you need to log in to the router via an ethernet cable linking the computer to the router -- one is usually provided with the router. Ethernet bypasses wireless security.

You still need to know the router login password and how to address the router via your browser. I suggest you read the Quick Start leaflet that came with the router or download the manual from the manufacturer's site.