Divide and Conquer
The first part of the organizing that is a must-do is also going to be the most mentally taxing. You will need to fight the packrat demons and actually go through all of the stuff in your garage. Divide everything into five categories: Used Regularly, Used Seasonally, Storage, Not Used, and Trash. Concentrate your "keeper" items into themed groups based on use and store them in this way once you've cleared or built space for them. For example, put everything you use for auto repair in one area and all of your sports gear in another area. Then, toss anything that's garbage and have a garage sale to get rid of anything you simply don't use anymore. Anything that is in good condition but doesn't get sold in the garage sale should be donated to charity.