1. Measure the box and find the dead zones
Write down four numbers before you buy anything: internal width, internal depth, floor-to-rail height and rail-to-ceiling height. Then measure the longest garment you actually own, hanging, from the top of the hanger hook to the hem.
That last number is where the dead space hides. A shirt needs about 40 inches of hanging height. Trousers folded over a bar need about 30. A long coat needs 68. If your rail sits at 60 inches and most of what hangs from it is shirts, you have roughly 20 inches of empty air under the hems across the full width of the closet — often three or four cubic feet doing nothing.
Do the same check above. Most closets have one shelf about 12 inches below the ceiling, which leaves a slot too tall for what people put there and too awkward to stack. And check the depth: if the closet is 24 inches deep but your rail is mounted 8 inches from the back wall, the front 10 inches is a corridor you walk your arm through.
Note the door while you are there, because it decides which fixes are available to you. A hinged door that opens outward gives you a whole extra storage surface on its inner face and full access to the opening. Bi-fold doors give you the opening but no usable door face and a track that limits what can sit on the floor. Sliding doors are the restrictive case: half the closet is behind a panel at any moment, so deep drawers and pull-out baskets work far better there than anything requiring a wide reach.
Sketch the box on paper with those numbers written on it. You are looking for three things: vertical air under the hems, vertical air above the shelf, and horizontal depth behind or in front of the rail. Every fix below targets one of them.