I had an echo at the front wall - I managed to eliminate this by placeing a couple of fabric panels 15 inches wide from floor to ceiling (almost) on the side walls at the corners that were slightly angled.
Once all the furniture/components are in place as Detlof says, walk about the room clapping - If you hear an echo then you'll need some form of treatment, like wall panels/ drapes, to eliminate it.
Eliminating the echo resulted in a less confused image
Be careful of overtreatment - it can lead to a dead and lifeless sound
Regards