Typically, when most people are on a de-clutter mission, they start in one room. If that approach isn't working for you, try to start with a category of stuff in your house or apartment.
Photo by Unnar Ýmir Björnsson
Farnam Street reviewed The Life-Changing Magic of Tidying Up: The Japanese Art of Decluttering and Organising by Marie Kondo. In the book, Kondo explains how we get distracted while cleaning up. We find the "hard" category to deal with and get stuck. She explains why a category approach prevents this problem:
People who get stuck halfway usually do so because they start with the things that are hardest to make decisions about. Things that bring back memories, such as photos, are not the place for beginners to start. Not only is the sheer volume of items in this category usually greater than that of any other, but it is also far harder to make a decision about whether or not to keep them.
Start with a category without a big emotional attachment (she suggests clothes). Go room by room and focus on getting rid of extra clothes. Focus your clean-up on categories you'll make easy decisions about. If you come across another category of stuff like pictures, put them aside and save it for another time.
The Japanese Art of Decluttering and Organising [Farnam Street]