Such good suggestions from Gransnet on decluttering. One small tip I can add is that once you pick something up to deal with it don't put it down again until you have dealt with it and you have put it in its final destination.
Keep a hole punch and lever arch files to hand for immediate filing of papers then have the triage bags or boxes ready for: dustbin, give away, recycle, sell.
Walk around your house and take a realistic look at what you do have space for. Plan out what the storage place is going to be for each kind of item for all the things that cause your clutter such as clothes, coats, footwear and accessories, memorabilia and photos, Christmas decorations, books, important papers and instruction leaflets, display of ornaments and knick knacks, office stationery, vacuum cleaner and cleaning materials, fabrics and hobby equipment, extra crockery, linen, cutlery and glassware, luggage and suitcases, newspapers and magazines. Then you will be able to step back and work out what you have got room for and this will help you decide what to keep.
If you have a lot of things that are kept just in case they will come in useful or for various projects just remind yourself that you really can't store them any more and that someone else will use them. Then be ruthless!
Also as I found with my Morher's house when I cleared it, piles of things that don't get used deteriorate, attract dust, vermin and insects and just go out of fashion and not in a good way so they end up going to landfill.