WPA keys are typically found on a label on the router or on a document from the ISP. If these methods are unsuccessful, you can attempt to find the WPA key through your web browser.
To attempt to find the WPA key through your web browser, connect an Ethernet cable between the router and the computer. Open Internet Explorer and enter each of the following addresses until a login box appears: http://192.168.0.1, http://192.168.1.1, http://10.10.0.2, http://192.168.2.1 or http://192.168.1.254. If the username and password already have values entered, press the Enter key. Otherwise, try using various combinations of "admin," "password" or leaving a fields blank. Once the system accepts your login credentials, look for an item called Wireless Settings or Security (the exact name depends on your router). The WPA key should be shown in this section.