S.O.S. – USB autorun a file

I am not tech at all. BUT I would like to have a file pop up on the screen, ready to open, when I plug in my USB. I read a few things – but just can’t make it work. Help!? Not sure what I am doing wrong – would really appreciate the “USB autorun/open for dummies version” of step by step instructions – did I mention for dummies (that’s me when it comes to tech) Thanks! Oh -and how do I get to the root of the USB??? Thanks again!