How to delete (hide) a mesh group or remove a mesh part

Hi sorry to bother but I tried this out but I got no idea of what I am doing! This my first attempt at blender so I do not know poo about how it works, and thought this tutorial would be enough but alas it is not so I got several questions: what is RMB and how do those squares appear? How do you make the squares over all as all that happens when I used the B button to select the verticles things was making it all yellow and then disappear, and what are these planes you are speaking of? I assume I might be not knowledgeable enough when it comes to this since this is my first time as I have no idea what anything of this is so I apologize for probably been a bit dumb, and the thing I am attempting to do might be a bit too large for a newbie as my first attempt, as I am trying to remove part of the cinema/movie theater building from growing together so that only the middle part of it remains, so it's not this huge building and instead smaller and can more easily be put in other worlds and fit more easily in with other buildings and so on