When you use something, move it to a separate drawer or pile. For example, kitchen utensils you use go into an empty drawer. At the end of a month, keep only what is in that drawer. Everything else gets donated. Clothes you have worn go into a special pile. At the end of the season, pack up everything that didn't make it into that pile.
Then tackle clutter. Allow yourself five decorative items per room. One box of sentimental stuff to save. Everything else would probably be better off donated and given to someone who needs it.
For toys, every new toy or book means one is given away.