Throw stuff out/give it away. Lots of stuff. If you have two of it, or don't really like it, or plan to replace it soon and won't need it till then, get rid of it.

Completely clear out some place, like a closet or a drawer or a shelf - do this by putting its contents in obviously inappropriate temporary locations, like on a bed, if necessary. Decide from scratch what belongs in this place. Put those things there. Repeat with the next space. If you don't have a way to efficiently use the space, buy an organizer of some kind suited to what you plan to put in. (Wire racks, drawer dividery things, bookends for open-ended shelves, etc.)