Start by gathering as many boxes as you think you will need, as well as tape, markers and packing material (i.e. foam shipping noodles, bubble wrap, packing paper or towels).
If you are storing documents, store them on a removable storage device, CD, or DVD and store them somewhere away from the storage unit.
Make a list of all equipment that you are storing and keep that in a safe place.
Any small objects should be stored in boxes.