Mess is more often not about dirt but about the number of things. The more of them there are, the harder it is to clean. Decluttering is a one-time effort that makes cleaning easier for a long time. Let us go step by step.
The one-year rule
The main question for every item: have you used it in the past year? If not, you most likely do not need it. This simple rule removes agonizing doubts and helps you make decisions quickly.
Go zone by zone, not through the whole apartment
Do not try to sort everything at once, it is exhausting. Take one zone: a shelf, a drawer, a closet. Small finished areas give a sense of progress and keep you from quitting halfway.
Three boxes: keep, give away, throw out
Prepare three containers. In the first goes what you keep, in the second things in good condition to give away or sell, in the third trash. Do not put it off with "I will decide later": that is exactly how clutter piles up.
How to keep order
To keep clutter from coming back, the rule "one item in, one item out" works. And after decluttering it is the perfect time to clean the freed-up space, or right away when you move, when excess stuff is especially in the way.