The easiest floor to make for your metal shed or plastic shed is a wood floor. The simplest foundation is pressure treated 4×4 skids sitting on concrete blocks.
In this case I used 2×4 floor joists 16 inch on center. Then you can either cover the floor with 3/4 inch CDX plywood or 1/2″ OSB. If you live in an especially wet area or termite prone area you could make the entire floor out of pressure treated wood. I live in the desert so we use untreated wood in our floors above the 4×4 skids.
But don’t make this common mistake that I see all the time. This floor extends beyond the walls of the metal shed so when it rains the water will run down the sides of the shed, onto the floor and back into the shed in the gap between the floor and the walls. And it is virtually impossible to seal this gap. The metal or plastic siding should hang over the edge of the floor an inch or so. Otherwise you will be fighting a leaky floor for the entire life of your metal or plastic storage shed.