Start small and collect all the stuff you didn't touch for more than 1 year - don't throw it away, just put it in the cellar and after a few months realising you don't need/want it give it away.
For me, category A includes things like the toilet plunger, flashlight, certain OTC medicines and first aid supplies, and the like. A few flattened cardboard boxes. A couple faded old towels. That step stool in the back of the closet. An extension cord.
Category B includes a minimum quantity of formalwear (tuxes may be easy to rent; heels that don't cause bleeding are another story), snow gear or an emergency generator in certain climates, perhaps certain types of cookware, etc.
As for category C... you don't use houseplants, but having a couple around can work wonders for your quality of life. The same goes for a poster/painting or two to liven your environment, or a few framed snapshots of family and friends. These aren't things that complicate your life - once you have them they don't cost money or significant time to maintain; there's no real pressure to keep accumulating more - and they're good for the soul.
Which is also why category D is important. Yes, you need to let go of things, and no, you shouldn't keep that takeout carton just because your ex touched it. But there are some things - that art project your kid made, or the cufflinks your grandfather left you, or the scarf your mom knit you the year before she died - that have value far beyond their utility. The challenge there is to decide which ones are really important.
But there is stuff most of us should be much more ruthless about getting rid of (living in a small apartment can help develop the ruthlessness habit; so can moving frequently). Top of the list for many of us is clothing. Stuff for hobbies we wanted to take up or just never get around to. Small kitchen appliances. Electronics and gadgets. Knick-knacks and collectibles. All the stuff you keep accumulating more of but that don't actually add anything to your life.
Hehehe.
Dad: Hey, I'm run ragged with stress and work and my life sucks. I think this simplify thing might help. Let's start by getting rid of the flatscreen TV and the couch.
Kids (much wailing and gnashing of teeth): No, no, nooooo!
Wife: You are sleeping in the spare room tonight asshat.
This scenario probably plays out a thousand times up and down the country.
This is like a solved problem (see any number of resources such as Your Money or Your Life by Joe Dominguez, Voluntary Simplicity by Duane Elgin, blogs by The Minimalists, Mr. Money Moustache, Early Retirement Extreme, ad infinitum...).
My advice, you can start by simplifying your own life. This frees up more time, money, you will be less stressed.
New scenario: You and your wife take a day off on the kids INSET day, go to the park, share a lovely picnic, chat together, take photos and take a walk around the lake...
Kids: Dad, this is the best day EVER! Can we do this more often?
Dad: I think I might know a way...
Wife: You are definitely NOT sleeping in the spare room tonight bigboy...