Installation grief

I downloaded a copy evaluation, then to be used by me (if I can figure this thing out) to test PHP.  I don't know PHP so I intended to use PHPStorm to runs tests of practice stuff but I am having a great deal of difficulty getting PHPStorm to work for me.  The "screens" do not necessarily match the documentation and the documentation assumes a decent knowledge of PHP.  For example, where do I find an interpreter?  I have installed XAMPP with joomla (I administer a joomla site) and I know and can see the XAMPP PHP directory but apparently PHPStorm can't see/find an interpreter.  I did try a simple "echo" PHP statement within a very small file and I get, "502 Bad Gateway" - all in all, PHPStorm is so rich in functionality that without detailed documentation I don't see how I can use it. IS there any documentation for someone really new to all of this?