Be sure to take EVERYTHING out of the garage. Decide if you are keeping, donating, selling at a yard sale or ditching each item. Keep items for a yard sale only if you feel it would truly be worthwhile and sell easily. After everything is out of the garage, be sure to clean the floor either by sweeping or vacuuming.
All the items you are keeping should be properly sorted so that like items are together. If you see duplicates, determine which is best, and get rid of the twin. Decide "zones" for each set of items. Reserve the premium (most accessible) space for those things you need most frequently. For example, we have our recycle bin right outside the door from the house since we usually have something to drop in each time we go out there. Remember to think VERTICALLY as well. You may decide to use shelving or as suggested by Consumers Reports, use the ceiling! "The ceiling has become the new frontier in garage storage, with systems designed to hold items as varied as hurricane shutters, big coolers and surfboards. Overhead storage is an economical alternative to a cabinet for large, long and relatively flat objects."
Be sure that everything is labeled, clearly and correctly. You can use a label maker, or just print with a wide Sharpie exactly what is inside a box, marking on all sides of the box. Another idea is to take photos of the contents and glue them on the outside of the carton. If you have several cartons of Christmas decorations, for example, take a picture of what is in each in the event you just want to find a particular item it makes the job a lot easier. Searching thru more than one box could be time consuming and frustrating, so marking boxes up front will save you lots of time and frustration in the future.