The easiest approach to cleaning the garage is to place a 55 gallon trash barrel in the center to throw away any junk that is lying around which will never be used again. Take all the unwanted items to donate to the local charity and get a receipt so the cost can come off your yearly personel taxes. Next tips is to hang up all sports equipment and toys plus all yard tools. Dust from top down including all shelves and remove all cobwebs. Follow this step by placing small items into labeled plastic milk carton style bins so it is easy to know what is inside. Place each storage bin on any shelving or off the floor.
Now is good time to bring in any garage cleaning supplies and equipment including a pressure washer. Any good janitorial service company will know to include a stiff push broom and floor squeegee. Do all the needed Windows inside and out. On a calm spring day open the over head doors and sweep then dust any shelves plus all heavy items you were not able to move like tool workbenches or cabinets.
Pretreat all the oil spots with a good grease remover such as Pour-N-Restore to remove oil stains from concrete. Follower up with a good moping with a general cleaner like Spic and Span and a half cup of vinegar. Pressure washing is the best and fastest method otherwise just use the stiff push broom and floor squeegee for cleaning grease. Floor safety is always a real concern.
Common-Business-Sense designates summer garage cleaning works out best in the northern states of the US. This will prevent the water from freezing before the floor dries. On the other hand it would be best to work in the early spring for any southern states to avoid any extreme high heat. Before replacing any items back inside consider using a storage shed for extra items.