Garden supplies tend to end up everywhere, especially if you do any potting in your shed. Always make sure to have a clean surface to work on. Old crates work well to hang from your potting area, where you can put small bags of soil, pots and watering cans. For seeds and other loose items, hang a ribbon and attach clothes pin. As you get small packets of seeds, use the clothespins to hang from the ribbon, taking down as needed. This keeps them organized and out of your workspace, so you can make the most of your gardening.
If you'd prefer that most of the work be done for you, then just select a shed from your local home improvement store and ask that it be delivered and assembled by the pros. But those who are handy may wish to save some money and order an outdoor storage building kit that comes with assembly instructions.