Do you use windows or mac?
House design software is easy to find with a google search, there are 100's of them. I use sweet home 3d. But it is only a house specific program, easy to use etc.
As for auto cad software these guys are probably one of the leaders
http://www.autodesk.com.au/ but generally their software is not free but they have free trials and the internet is full of ways to keep using those trials if your not doing it commercially.
I used 3d studio max when I was a windows user, I now use Auto Cad on mac. Both are very powerful programs.
3D Studio Max
AutoCAD
Then there is free software like google sketch up. Very basic and lots of online tutorials. But it can be very powerful too if your willing to add on to it.