You open your browser and type in the address of the router, it will be in the instruction manual or on the bottom or side of the router, then when you hit enter a login screen will open and you put in the default user name and password of the router, again this will be found on the bottom of the router or in the instruction manual. Once in the routers internal settings you go to the security tab and that's where the security key can be made or viewed.
Any of the login information can be obtained by going to the web page of the manufacturer of the router and you can get the generic login information.
Once you are in the routers settings and you decide to make a new security key or password you should write it down and keep it in a safe place because once you make it your the only one that knows it. Always save your settings and changes in the router.