The garage is more than just a place for parking a car for many people. If you're in the habit of using your garage for tool store, a safe space for seasonal items, or as an over-flow attic, things can get out of control quickly. Fortunately, you can learn to address the mess, get the proper organization tools for the job, and choose a method of organization that works best for the stuff you've got.
Get rid of the useless stuff. Whatever you end up putting in the "unusable" pile, get rid of it as soon as possible. Don't take it to the dump "next time you get around to it," do it right now. Clearing out the clutter is the only way to commit to a more organized and efficient work space in your garage. If you don't need it, don't keep it around to take up premium space.