Greenhouses, Sheds and Sun Rooms - HELP! Converting/insulating a shed.

Thanks so much for taking the time to help me out. Ok so I have a pre-fabed shed that came with my house when I bought it. It is almost brand new. I want to basically convert it to a temperature controlled "extra room for my house" It will be a craft/music room for my wife and I.

I have put a couple of pictures of it on my website that may help you see exactly what I have if you think it might help. http://www.kurtisdonnelly.com/shed.html

So I am going to insulate it and then put up sheet rock or some sort of plywood, then add a window AC unit and some sort of electric heater. It is already wired of course.

Here are my questions!
1)I need help with insulation. I am using R-19 as is specified by my local state recommendations on the "walls". As you can see from the picture the walls themselves are only like 5 feet tall and then the roof starts. What do I need to do on the ceiling part? I have red stuff about leaving space between the insulation and the plywood roof. I have also read about different types of vents, but I have no clue what that means. I assume I put the faced side of the insulation toward the inside for the entire building?

2) The floor is just plywood. It will be very hard for me to get under the shed itself to do anything underneath. What can I do about the floor?

3)I will put some sort of stripping around the doors and windows, will that be enough?

4) what would you suggest for covering the walls? Sheet rock, plywood, or some other type of board.

5) Finally, what would you suggest to heat the inside. I would like to be able to leave it on a bit during the winter to protect stuff in there from freezing.

Thank you so much for your time, I really appreciate it, and my wife will to when she sees what I do for her! Any other tips you can provide would be very much appreciated!

Thanks!