Figure the area of the walls and add them together. 16'x8'x2 + 20'x8'x2 = 256 sq. ft + 320 sq. ft. = 576 sq. feet surface area for the walls.
A board foot is 1' wide x 1' long by 1" thick and is usually the measurement when buying hardwoods.
When building a building, you need to think of 4'x8' dimensions, so that you can sheath it with 4'x8' sheathing. The short sides will take 4 sheets, the long sides will take 5, so you will need 18 sheets of 4x8 material to cover the walls (not counting cutouts for doors and windows. I would get a couple extra just in case. The number of sheets for the gable ends will depend on the pitch of the roof... but you can calculate that as well.
Then siding would normally go on the outside of the sheathing, depending on type.
Good luck! it's too hot here to be outside (for us wimps!)