Lets see....me personally, I would first go through and pick up any trash and make a trash pile. I would then throw out anything I can visually see, with out getting in to boxes yet, that you know you don't need anymore. Then I would try to stack and organize all of the boxes on one side of the room, to clear up space to sort through the items in the boxes. Then I would go through each box one at a time. For every item you take out, decided immediately if you want to keep it or get rid of it and do it right then. Put in either your donate pile or trash pile. I would maybe try to have 2 empty bins or boxes as I go to keep like items organized, such as Christmas items in one, fall items in another and so on.
Good Luck!!! I know it can be daunting, my garage can get this way!
Jessica, Single Mama to Lailah(4) and Christopher(2)