First, I am assuming that your garage is attached to the home and you will have a entrance built to join the two or something.
If it was my home, I would make the floor the same height as the existing home. I would use pilings and build the floor joist system just like a deck with joists 16 inch on centers. I would use 2 x 8's joists on 6 x 6 beams.
I would make one complete level floor and just go from there.
Diagram out your bedroom, your bath, your study, and all with spray paint, then erect the walls and this should be alot of fun.
Check out all local permit situations and get your ducks in a row.