Sketchup can be really interesting for the floor plan and even to put your idea in 3D. The software is really user friendly, even for people with no CAD experience.
After that, if you want to go further, you can render your design and make it look like a real picture.
I personally work with AutoCAD for beginning ( the 2D part) then go in Sketchup Pro for the 3D. After, I render the 3D model in Kerkythea ( free rendering program).