Go through the attic and remove everything that you can easily carry to another room. This will give you space to clean, and will make it easier to get rid of things that you don't want to carry back.
Thoroughly sweep the attic, removing all dust, cobwebs and dead bugs. Wear a dust mask to protect your lungs if the air gets too thick.
Wipe the windows and floor with damp rags to remove any remaining grime. You don't have to scrub the attic, but you do want to remove as much dust and dirt as possible.
Remove any pieces of furniture that you don't plan to use. Have a friend help carry them if they are too heavy to move by yourself.
Go through your items with several heavy-duty trash bags and throw away any obvious trash. If you have items you stored years ago with the intention of having them fixed, ask yourself realistically if you ever will. If the answer is "no," throw them away.
Go through containers of clothing and ask the same question from the previous step. If you haven't worn it in five years, you probably never will again.
Organize the remaining clothes into boxes by the season that they are worn. Mark the boxes on multiple sides with a fat permanent marker.
Go through your holiday decorations, throwing away old and broken items and organizing the rest into boxes by holiday.
Organize other items such as books, toys and camping supplies into boxes following the same system.
Put the organized boxes back in the attic. Place them in sections with aisles between them to make access as easy as possible. Items used more often should be toward the front, while seldom-used items should be near the back.