How can I find WPA key? Thanks

Since you created it, or whoever setup your router did, we can't tell you how to find it. However, if you don't know what it is, you can reset your router back to default settings and recreate one.

If you are connected to the network then u can use this if u are using windows
WirelessKeyView: Recover lost WEP/WPA key/password stored by Wireless Zero Configuration service (http://www.nirsoft.net/utils/wireless_key.html)