If you'd prefer that most of the work be done for you, then just select a shed from your local home improvement store and ask that it be delivered and assembled by the pros. But those who are handy may wish to save some money and order an outdoor storage building kit that comes with assembly instructions.
Before installing storage units, lay down cinder blocks, concrete piers, a concrete slab, or at least crushed stone. But don't stop there. Make sure the supports underlying the floor are made of pressure-treated wood.