If you're looking to get more organised, it's really a whole journey to having a full system and the habits to maintain it. However, if you're just looking for that quick hack that will give you the most benefit with the least effort, start using a filing system. Buy a ton of manila folders and a filing cabinet, and start using it for literally everything that will fit. Don't leave things around the house in places where you'll "just remember", and definitely don't pile things on the desk, even if you have a piling system ("this is next week's bills, this is letters from Grandma, and these on the floor over here manuals for household appliances that stopped working five years ago"). Your goal in this filing-obsessed situation is that the first response when you wonder "where is such-and-such?" is always "filing cabinet". If you can't fit an item in there physically, you find a place for it elsewhere, then write down where it is in your filing system. It should be like the library card index of your house.
Mokalus of Borg
PS - My filing system is not quite as good as this.
PPS - It needs some maintenance.