Can someone please tell me how I can model a simple rectangular room with a doorway?
Welcome to blenderartists.org to get started with blender take a look at the manual and the tutorials on the blender wiki
http://wiki.blender.org/index.php/Manual
http://wiki.blender.org/index.php/Tutorials#Introduction
There are nice tutorials for modelling a houses on vimeo
Part 1: http://www.vimeo.com/785249
or you can go to the tutorials channel at:
http://www.vimeo.com/BlenderTuts/page:22
Hope that helps!