help with vapor barrier/insulation for shed/workshop

It has been a longer wait on building my shed than I expected, due to financial issues. I have finished gravel foundation and have the floor ready to go. I have a question about vapor barrier/insulation. This 8X10' workshop/shed is going to be built with all pressure treated 2X4's (except for siding). The shed will have two 2X2' windows (North and South) and one door on Southern side. There will be a ridge vent on roof. Eventually I would like to carpet the floor. The shed will be set on four skids about seven inches from the ground. I want to heat the shed most of the winter as it gets cold here from about middle to end of November to end of April. My concern is what kind of vapor barrier I should use under or on the floor (or both?) to keep the floor somewhat moisture free. I am also trying to figure out what kind of insulation barriers to use for the outside under the (painted or stained) 4X8' siding and inside over the R13 insulation and roof. I would like a loft but it might be more expensive to insulate then it would be to insulate a flat ceiling overhead and leave top part somewhat open. I have done a little research and there are different kinds of vapor barriers, reflective, polyethylene, etc., and at a loss to choose any or where to get them where I live. Mostly I have shopped at Lowes hardware, but the only types of barriers they have are in bigger rolls then I need or very expensive. I realize I am asking many questions. Any help would be much appreciated.

Paper back insulation is the cheapest since you buying insulation anyway. Plastic sheeting is good to, just don't use both at one time. Nothing wrong with one on the dirt if your foundation is enclosed. If it is open I wouldn't bother. Just put some against the floor and then the insulation.

Oh yea, place the vapor barrier against the heated side, not the outside.

Thanks

Thanks Jeff for the advice.