Abeer khan

I am near to starting a garage conversion. However I am receiving different / confusing information with regard to the stud walls and a Vapour Membrane. Q - Is the vapour membrane attached to the stud work on top of the insulation on the inside of the building before I plasterboard or is the VM attached to the stud work on brick side of the stud. I also intend to insulate the roof of the garage, Q - when I do insulate do I make an air gap or is the insulation pushed up tight to the OSB underside of the garage roof then attach a membrane before I plaster board ? Any advice would be appreciated, thanks in advance.