Check the electrical wiring on the walls and the ceiling. It is much less expensive to reroute wiring before the original walls and ceiling are covered and painted.
Attach drywall sheets to the garage's ceiling joists, or attach ceiling tiles. In some garages, the ceiling can be nailed directly to existing trusses; in others, it may be necessary to install new joists. If you install drywall, coat it with joint compound, and then paint the drywall.
Nail or screw drywall sheets to wall studs, or use prefabricated veneered sheets on the walls. Coat drywall with joint compound prior to painting.
Install the flooring material. If the original garage concrete floor is not completely dry or level, seal it, apply a moisture barrier and build a plywood subfloor on top of the concrete. If the original floor is completely dry, you may attach finished flooring directly. Flooring options include wood, tiles, vinyl sheets, laminate and carpeting.
Install electrical outlets and switches, a thermostat and plumbing fixtures.
Install exterior siding on the outside of the garage to disguise the conversion. Options include resurfacing the entirety of the exterior walls or patching them so they match the rest of the home. Also consider taking out the original driveway and installing landscaping, including plants.