Paperwork utilities and banks online
payslips keep for a year until get P60 ( if you are self employed you need to keep all related paperwork at least 7 years)
Insurance current one only
receipts for important stuff until guarantee runs out
Certificates birth marriage divorce exams house deeds keep originals and ideally a spare copy elsewhere, if dealing with a death get at least 2 copies ( ideally these together with passports and driving licences in a small fire proof safe)
There is a fine balance between enough stuff, too much and not enough, e.g you do not want to have to go and buy new sellotape everytime you need it but you do not need 6 rolls
Clothes only what fits and is worn regularly, the only exceptions I would make are a wedding dress or expensive formal clothes or specific sporting clothes for a sport you still do, if you go skiing every 2nd winter keep the stuff if you haven't been for 10 years sell or bin
Food enough for a week or two in case you get ill, break a leg or bad weather, medication at least 2 weeks worth
Things like books /art/ ornaments etc depend on space if you have room for 3 bookcases that's fine if not you need to be picky
Don't keep something because Aunty Flo bought it for you 22nd birthday and you don't like it
Don't keep broken/ damaged/ torn stuff unless you really honestly can repair it and reuse it and will do so in next 2 weeks
Photos don't keep ones when you don't know who the people are in them or where they were taken, historic ones keep but not necessarily all of them and carefully write in pencil on the back who, when and where so your children know this is the only picture of their great great grandmother that was born in 1886 for instance
Keepsakes for my DD I have first babygro and dress, her first shoes, hospital name tag and one hand crocheted blanket set which she can have as heirloom it's beautiful, I have kept a very small selection of toys a small box useful for visiting families myparents kept a few of our toys so DD has my old lego and board games to play with when we visit also a few classic books like Rupert etc I photograph the best pictures and keep in a file I display the originals in a certain place so when another comes one existing one is removed, Keep school reports but not their maths exercise books
DD is much better at getting rid of toys she has outgrown now she is allowed to keep any money they make or decide which charity she wants them to go to