3 Answers

2"x6" floor joists, pressure treated lumber. Double plate around perimeter. Floor should be 3/4" also pressure treated. You can apply 2 coats of "Thompsons" weather sealer instead of the pressure treated lumber. Don't forget to place two supports in center of floor. Use screws instead of nails. Wall studs should be 2"x4" @ 16" on center. Exterior can be OSB ( 1/2 price of plywood) rather than plywood but must have some type of siding or cover on it.

As a base, you may build the shed frame as described above, upon PT 6x6 timbers, which have been leveled over concrete pylons poured into sonic tubes to a depth of at least a foot, and leveled to each other.  

use treated wood.