I also recommend Google SketchUp, there are no real limits to this simple software, I say simple but you can create the most stunning photo realistic renders. From basic line drawings to scale or apply a measurement on screen to enable the printed version to have dimensions.
The plugins available allow you to create pretty much anything and can provide a perfect presentation tool when chasing projects.
I quite often find this useful when there is only 2d, plan and side elevation drawings for your liftoff and so with a quick sketch all can become apparent quite quickly…..all in iso and perpective of course. You can even export in most CAD formats DWF and the like.