Describe the roof structure or ceiling structure if there is a room above the garage.

You will probably be required to insulate the walls and perhaps the roof or ceiling

Congratulations on your project. You asked specificially about ceiling treatment, but there are also a host of other issues to convert an "exterior" space to a habitable one. Here's some thoughts about garage conversions:

Floor slab: Is it solid with no cracks? Is it at house finish floor level or are you OK with a step down? Is it insulated below the slab and on stem walls? Are you in a climatic zone where winter coldness is a factor for the concrete slab? If so, floating a floor on sleepers (with insulation) above the top of the slab is an option.

Walls: Will additional sealing of cracks and insulation be needed? Will new gyp board and trim be needed in some areas?

Ceiling and roof: What sort of roof structure do you have? How will you insulate and vent the roof to prevent heat loss and moisture condensation/mold? You will likely be better served to have some sort of seperation between the habitable space below and the insulation/moisture barrier above which is usually not very visually attractive (what a ceiling usually does). Although I hate the look of a suspended ceiling, it may offer one inexpensive option for a new ceiling if the existing roof joists or trusses have uneven bottoms.

Systems: How will you heat and cool the new space? Will you need to run new HVAC ducts, control wiring etc.? Will you need new electrical circuits, controls and a secondary panel? Need water and drainage?

I'm sure you have thought through all of this, so pardon me if this is redundant. Good luck!

I don't really know what you are trying to do. I don't know how the garage roof is constructed or what the climate is so I'm having just as hard a time as you are.

You ask for pictures to share but you don't offer any.