If its an ordinary wooden shed then there should be no need for a concrete base as the shed would probably have its own floor. A row of blocks set in the ground around the perimeter and maybe an extra row across the middle, under a timber that's supporting the shed floor, should be quite adequate. Let the blocks sit up maybe 2 inches over the ground level to keep the shed clear of surface water/damp, which should prolong the life of the shed. No need for mortar with the blocks, just make sure the ground is reasonably solid. A concrete base is overkill, IMHO.