How can I insulate a shed floor?
For the R-Value of your walls and roof, it will depend on the depth of the framing, but assuming you're using 2x4 studs in your wall, then R18 is pretty standard.  For your ceiling, again depending on your framing, R30 or R38 is typical for batts.  Remember that you may be able to squeeze a higher R-Value batt in between the framing, but compressing the fiberglass effectively reduces the R-Value by creating thermal short circuits.

For the floor, install the rigid insulation, then the vapor barrier, then the floor boards.  Here's a pdf which talks about vapor barriers: