Needless to say, you'll need insulation, regardless of where you live. When dh converted a shed for my sewing, he found some flooring on sale, at either Lowe's or Home Depot. It's just vinyl 'planks' but they look nice, are easy to clean, and my chair rolls effortlessly. You'll also need to work on the lighting. We chose 'daylight' fluorescent lighting, which works for me.
Can you replace the garage door with a wall and door? A LOT of heat/cooling escapes, through a garage door.