Check the electrical system. Make sure there are enough 20-amp circuits to handle any electrical appliances or power tools that will be used in the space. Check your local zoning codes and have any electrical work done professionally or it will not pass inspection.
Install ceiling storage units. Measure the space between the ceiling trusses to determine the size you need.
Put in a second door to the yard, besides the one for the car, even if there is a door into the house. Easy access to the yard means chairs and other items can be brought out easily.
Make sure there are enough windows and they are large enough to let in sufficient sunlight. Install insulation and dry wall. Place any work benches and then work the storage cabinets and wall units around them.
Repair any cracks in the floor. Paint cement floors with special cement paint or install a floating or rubber tile floor. Both are accepted flooring options for a garage.