Many sheds are successfully built without a concrete foundation. I have an old horse barn in my backyard that has been standing since 1959, with a foundation comprised of concrete block in the corners, and a few blocks in the center. I am not sure how you "know" you have to lay concrete, but there are plenty of folks on this site who have sheds of various sizes on types built on a wide range of foundations, including PT wood, concrete block, stone, posts etc.
Perhaps a better way to approach the question of foundation would be for you to explain exactly what you are thinking of building, size, type of floor, type of framing, type of material you want to store inside, your location, and the type of soil, and see what folks on this forum think might be a good, low cost foundation type.
And you may want to check with your building inspector, there could be local rules regulating foundation type and depth. Especially important if you need a permit.