If you have a DSL, Verizon FiOS, or AT&T Uverse Modem/Router combination the WEP Key for your network is typically printed on the bottom of the device.
If you are using a standard wireless router, the network password was setup while installing the router. You may need to login to your router's web interface to find out what your password is if you can't remember. This information is typically under Wireless Network > Wireless Security.
If you are unsure of your password or how to login to your router, please consult the router's manufacturer, your internet service provider (DSL, FiOS, and Uverse customers) or your router's user manual.