There are too many variables in the design for the shed. It is easy to say that all you require (but check your local code requirements) is a 2x4 wall sheathed on both sides by 3/8" plywood. You can attach your siding on this. You may want to fill the spa
ce within the studs with batt insulation, to keep the heat down in the summer. Your roof could be designed the same way, with the use of 2x4 lumber, 3/8" plywood, and batt insulation.