Whoever setup your wireless router configured it with some form of security and set a key or passphrase. What router do you have and who set it up? If you don't know the key and can't get it from whoever set the router up, then your best bet is to reset the device to factory default settings, log into it and re-confirgue from scratch. There should be a recessed Reset switch on the back somewhere, if you press this for some number of seconds (different for different models), the router will reset all settings, including your wireless setup and password/key to the original settings. Without more specific info, I can't be mre precise.
If this device is one of the new Xfinity Wireless Gateways (SMC model SMCD3GNV), then the default access URL, admin info, and wireless info are on a sticker on the device itself. See this FAQ for how to configure that device:
Xfinity Wireless Gateway Setup
