Insulated Internal Cladding of an outside Garage

[PeterJCW]

I am in the process of refurbishing my outside garage, which has one external door and a typical normal garage door. I have read about the problem of damp and am concerned. I am proposing to to put Self leveling compound on the floor to seal the bare concrete (45 years old) and then cladding the walls with shiplap. I think that the easiest insulation would be foam boards,. I am wondering how to damp proof it in order to prevent the project going pear shape with lots of mould etc. The roof is flat and uninsulated.
I have looked on lots of sites and got confused as to the best solution...
Any suggestions would be greatfully received. Thank you
Peter

[aeromech3]

Yes it is confusing, are you just going to use it as a garage or will there be heating? What is the garage material, walls and roofing and how much height have you to play with for the floor compound thickness, as a vapour barrier first before laying would be a very good idea?