Start by getting everything out from under the sink! Are you sensing a theme here? It’s much easier to sort, organize, and start fresh when you have an empty space in front of you.
Only keep items that pertain to the kitchen. The bathroom cleaner should probably go in the bathroom, and lets put the bug spray in the garage or basement. Kitchen related cleaners, dish detergent, and trash bags are all just fine to stay.
Typically there’s a lot of height to work with in the under-the-sink cabinet. I use a wire shelf to take advantage of this vertical space.
Do you save plastic bags? They can be great for collecting trash in the car, walking the dog, small trash can liners, and much more. I love using baby puff snack containers to keep plastic bags handy (they fit in cupholders!), but you could use an empty baby wipes or cleaning wipes container, or buy a container specifically made for the job.