How to achieve a clay render look?

Well, the first thing I can instantly detect when comparing your SS and the example is that you have no shadows.
I would start with the adding of a shadow generator with soft shadows.

Edit: you do have shadows on the environment but I cannot see them on the furniture.
You should try to work your lights and shadows and for the smooth look effect, you can work the rendering pipeline and try SSAO/SSAO2 as suggested above.