Set up a workbench. This will be where you house your tools, potting soil, hoses, plant labels, and so on.
Hang things around the perimeter of the warehouse. Use hooks to line the outer rim. This will prevent you from bumping your head on hanging baskets or tools.
Place garbage cans in corners. Plastic bins are great as a water proof option, while metal bins will be better for keeping mice out. Bins are useful for storing fertilizers, but are also useful when caring for your plants when dead heading.
Place additional shelving or stacking bins along the wall for smaller materials. If you can hang it on the wall, then that will allow for more floor space.
Store your plants to the south of the warehouse in the winter for maximum sunlight. Hanging baskets and taller plants should be towards the north end.