Everyone is going to have a different way, I always did 2x4's 8" o/c with 4x4 pt skids going the opposite way 24"-36" o/c. All PT until the wall studs, you won't regret a 3/4 PT plywood on the floor, I would also glue and screw all the components of the floor. The Floor is the most important part of your build, on such a small building doing 2x4's 8 o/c with skids, and 3/4 floor all PT will only cost you a few bucks more...
With the floor this way I have parked my cat skid steer on it, and that thing is HEAVY, all the doors still open and shut..