Have you tried Google sketch


This is the easiest and the best free 3d program for novice and even experts. The photo you see is what I made in just 15 minutes with the program and that was in the first interaction with the program. The photo is the plan of the shop i work in. the program is around 35 Mb download and runs perfectly on my 300 MHz 256 Mb ram system. There is even a tutorial in the begining, that is enough to make you good with the program.

So go ahead and give it a try.