Build a Testing environment website for running website

Hello The goal here is to achieve a testing environment for our website. There are several things we require this to work. 1) On demo website we should be able to try out every new customisations. We need this test new modifications before making them live on main website and when finalised we can apply those changes to our main website. 2) The demo website should always be an exact copy of our live main website. Every final changes made on live website should automatically be showed up on the demo website. So in future if we make any changed to live website we can try those changes on latest new website not the older one without any new changes. Any suggestions to make it better are welcome.