As with all organizing, decluttering is step ONE. As you unload each item from the attic, decide if you are keeping it (meaning it brings you great joy or serves a purpose) donating it or tossing it in the trash. If the job is going to be huge, will you need to rent a dumpster to put in the front yard to fill as you encounter all that trash? Also, decide where you would like to donate things, and be sure to segment each item into one of these categories: ditch, donate and keep elsewhere in the house (i.e. doesn't belong in the attic and needs to be put away elsewhere in the home) and finally those things that will go back into the attic. When planning a good block of time to take on this chore, you might want to enlist the help of family or a friend as there may be heavy things to be moved. When we did this at my in-laws (after many many years of being untouched) we had all the siblings there which made the job go quickly when there are 7 grown kids in the family!
Once you have completed the decluttering, sweep out the area and determine zones within the attic to keep your holiday decorations, skiing equipment, extra luggage etc. Be sure those things you use most often are easy to access. Any "bins" that go up there or garment bags must be clearly marked on all sides for easy identification. Once you are done, stand back and enjoy your hard work and take pride in what you have done! You will see it also will add more calm to your life...just what we all need.