If you have a wood-burning stove or fireplace, you need a place to store the firewood until it's needed. Outdoor storage works, but it's wasteful -- some of the wood inevitably decays before it's used, plus it may get wet or snowed under. The answer is a woodshed.
Check with your local council's planning department to find out what planning permission is required. Often, sheds with a floor space under a certain limit are exempt. But there may be provisions that prohibit certain construction methods. Better to find out in advance and plan for it than to be fined, forced to tear down your work or suffer additional expense to bring a building into compliance.
Calculate the size of the building you will need. This will be based on how much wood you need to store.
Choose the site for the building based on elevation of the land. Don't position it in a low area that will collect water or get muddy when you're hauling wood to and from the shed. Decide if you want lights or electrical sockets in the shed and how to route power to the building. Also consider the distance you will need to haul the wood from the shed to where it will be burnt.
Decide what sort of construction method to use. A pole barn style of construction -- in which sheet metal siding and roofing attaches to a wooden interior framework -- is probably the least expensive, but will require a good deal of interior reinforcement. Stick-built -- using timber such as "two-by-fours" and "two-by-sixes" to construct a framework on a solid foundation -- will cost more, but require less interior reinforcement.
Pick a type of floor to install. Don't use a dirt floor. The bottom layer of wood will stay wet and as it decays, the stacks of wood inside are likely to topple. Gravel is better, but not as good as concrete. Concrete will cost more, but is worth it in the long run.
Vent the building well. You want the wood to dry and, as it does, the moist air will need to escape. Include a series of louvres or other wall vents near the floor as well as gable-end or roof vents. This will cause natural circulation as the warm air inside rises and pulls in cool air through the wall vents.
Brace the shed thoroughly. Inevitably, wood piles will shift as the wood dries, occasionally toppling the pile or causing chunks of firewood to roll off and hit the walls. Without interior bracing, such mishaps can push out a wall.
Install a door or doors large enough to allow easy access to the shed by truck or trailer.
Place screens over vents or louvres to keep rodents out.