Cleaning a very dusty garage
Buy/rent a shop vac. Start from the ceiling and work your way to the floor. Repeat the process as may times as you need until you are satisfied with the results (allow time between repeats so that disturbed dust is allowed to settle).

Once done, sift through the contents of the shop vac's collection canister and remove any thing you want to keep.

If you're concerned about breathing in the dust while cleaning, get a good respirator/dust mask and use it while vacuuming.