A solid concrete slab provides a very stable and strong foundation for a storage shed. Solid concrete has the advantage of providing a floor as well.
Another foundation that provides a solid base uses concrete footings and tubes into which concrete is poured. Beams that provide a foundation for the floor are attached using post bases.
Concrete pavers and gravel are popular low-cost foundation options. These options require the area where the shed is to be very level. Shed manufacturers sometimes offer pre-made foundations or floor kits that typically are convenient for the a person with some working knowledge to assemble quickly.