Roof
Try first with the polyroof tool. See attached. This will place a roof bounded by your contiguous walls. It uses the magic wand to locate the perimeter - It follows around and makes a great job.
Walls
You must specify the actual wall thickness in the Wall tool. Each pen has a controllable thickness. But on-screen you can draw in real thickness or hairline.
Render
Use camera tool to place a camera on the plan in a reasonable place relative to your building. SELECT the camera. F3 for a view.
Every one of your questions indicates that you should thoroughly study the documentation and have a friend show you a few things....
Dwight Atkinson