The first thing you need to build an 8-foot by 16-foot shed is a building permit. The second thing is help: This is a major construction project and not a one-person task. You also will need a tape measure, shovel, gravel compactor, framing square, circular saw, wood stakes, builder's twine, a good hammer, plenty of framing nails, a ladder — or two — and possibly other tools, depending on the shed type.
A good shed starts with a foundation. You will want a concrete slab for an 8-foot by 16-foot shed. That requires a shovel or excavator to dig the foundation area; forms to frame it, either rented metal ones or forms built of 2-inch by 4-inch lumber secured with wood stakes; medium gravel; a compactor to solidify the gravel base; concrete; a rake to spread the concrete; a long board to screed, or smooth the concrete; and a long-handled finishing tool to apply a final finish. You will also need rubber boots to wear as you're wading in the concrete and spreading it.
Walls require 2-inch by 4-inch framing lumber, pressure-treated for plates that rest on the concrete slab. Side walls need 16-foot plates, end walls 8-footers. Interior studs will be 8 feet; the number will depend on spacing, whether you're setting the studs 16 or 24 inches apart. The walls also need cap boards to tie them together: 8 and 16 feet for end and side walls, respectively. You also need a 2-inch by 6-inch header board to frame the top of a door opening.
A gable roof, the most common style, typically uses 2-inch by 4-inch rafters or prefabricated roof trusses. You can also make your own trusses by cutting rafters and securing them with metal connectors called gussets. The length of rafter boards will vary with the pitch or slope of the roof. A 4/12 roof, which rises 4 inches per foot, can use 8-foot rafter boards; a steeper 6/12 roof requires 12-footers.
A roof also requires decking, typically 4-foot by 8-foot panels of plywood or oriented strand board (OSB); roofing paper; an asphaltic waterproofing material; and shingles. Shed roof styles and coverings vary. You may use corrugated metal, aluminum or galvanized steel or even vinyl panels; these all come in varying widths and lengths.
Siding for walls varies. You can use corrugated metal, exterior-grade plywood, some sort of wood planks or OSB sheathing covered with vinyl siding. Metal or plywood panels are easiest to install. Siding planks can be tongue-and-groove type in various styles or clapboard with planks overlapping. You also can use cement fiber board, which resembles wood planking but is made of a cement material that resists rot.
You need some type of trim with any wood siding, typically 1-inch by 4-inch boards at the corners and tops of walls, and 1-inch by 2-inch batten boards to seal the vertical seams on plywood panels. Vinyl siding comes with corner and top trim pieces, but you will need steel starter strips and other special fastening elements.
Buying a prehung door will be the easiest solution for a door. An 8-foot by 16-foot shed usually needs a double-width door, either a style that opens to two sides or a sliding panel that moves across the end wall. Prehung doors come installed in frames that are secured into the rough openings in the wall. Sliding panels have top and bottom tracks that are fastened to the wall.