If you have never worked with 3D software, and don't want to spend many weeks to get the hang of it, I'd suggest you to use Google SketchUp (free version):
http://www.google.com/sketchup/download/gsu.html
It's the easiest to use for beginners in my opinion.
Otherwise there are quite a few free 3D programs that are not extremely difficult to learn, but still you have to have some patience and time to learn to work with them.
As mentioned before you can check out this thread for a list (which contains free and paid ones):
http://www.adventuremaker.com/phpBB2/vi ... php?t=1264
From the free software, I find (apart from Google SketchUp)
Art of Illusion and
Deled CE rather easy to learn, but it's also always a personal choice and taste - and I think that everyone would suggest a different one.
So if you want to put time and effort into it, you can try some of the free ones for yourself and see how you get along with them