Remove all of your belongings, including your cars, from the garage. Start with a blank slate. This provides a good opportunity to go through your belongings and throw out or give away anything you don't need.
Pressure wash your garage floors with trisodium phosphate (TSP) cleaner. Most garage floors are simply concrete slabs. After it is clean, you may also choose to acid stain your concrete slab to add color and warmth to this otherwise cold space. Another option is to add a durable and attractive epoxy coating on top of your existing floor.
Hang drywall if your garage walls are unfinished. Some garages are left with the studs exposed, but by finishing out the walls, the space will look more attractive and provide a surface on which to hang and organize your belongings. Consider hanging plywood instead of drywall for a more durable finish, if you are concerned about bumping and damaging your walls. At the least, provide a plywood backing behind drywall where you plan to hang shelving. You may also hang drywall to finish the ceiling plane.
Prime and paint all of the new drywall surfaces. White is a common choice for garage interiors because garages often lack windows and it makes the space feel larger. However, white walls will show smudges easily, so consider a light putty color or beige.
Install a shelving system on which to organize all of your tools and other belongings. There are several different types, from basic open shelving to special peg systems that provide a place for every tool.
Hang new lighting fixtures so you can see to find your newly organized belongings and take better advantage of your garage as a workspace. Industrial-style lighting fixtures are affordable and adequate, although you may choose a nicer fixtures if you prefer.