How to I cut a specific part of an object in order to have 2 separate objects? I'm completely green to Blender.  Whilst I've used 3DS Max in the past, I haven't got a clue how to use Blender and it isn't striking me as very comprehensive, so I thought I'd reach out.  I've downloaded a public domain telephone box through STLFinder which someone expertly made but as one solid object. I've successfully finally managed to rescale it to the size I wanted and cut a hole in the base, but that's as far as I could figure out. 
 I'd really like for the door to open then 3D Print the 2 sections and assemble it.  In edit mode there are countless thousands of edges and I'm feeling overwhelmed.  I've looked at various forum posts and youtube videos which talk about cutting an object clean in half, but I don't want half, I only want to cut out the door as an entirely separate object so I can design some sort of hinge for it ... but that would require cutting 3 dimensionally.  I wondered if a modifier like Boolean would achieve this and if so, how do I go about it?

Big thanks to all who responds!
I would try to press "L" key in edit mode with cursor over some part of the model. This selects "Island" in case the model is not merged together. if it is merged, you can try to select one face somewhere in the middle of the part you want to separate then press "ctrl +" to start adding faces to your selection. To separate your selection you can use "P" key then "separate selection"