Not sure, but you could start by checking any documentation that whoever set it up might have left regarding the security / passphrase. It may also have been left at default and perhaps that may be present on the bottom of the router itself.
Failing this, just use your router's browser interface and look for something under the Wireless Settings, referencing Wireless Security and you should either be able to see it or if you wish, alter it to something you prefer.
Regards
Randy