The most important tools you will need to construct shacks are the following:
Floors: Your basic floor, Shack Floor, is found in Structures > Wood > Floors. You"ll need this for any type of shack, wood or metal.
Walls: Walls come in two flavors, "Wood" and "Metal." Your basic Shack Wall will snap to any floor you place, internal or external.
Doorways: There are special Shack Doorway pieces (Structures > Wood/Metal > Walls) you"ll need if you want to place a door to your shack rather than leaving it open to the empty air (also an aesthetically pleasing option, but inadvisable on upper stories, as your idiot settlers will plummet to their deaths). The door will snap right into the empty place.
Roofs: There"s a wide variety of wood roofs under Structures > Wood > Roofs; not so much for metal under. They"ll snap to even one wall.
While the game will permit you to place walls without a floor, I strongly recommend placing a floor first (or a foundation if you"re working on uneven ground). The floor gives your walls something to snap around; otherwise, they will snap in a straight line into eternity.
While the game also does not prevent you from mixing wood and metal shack pieces, I suggest you stick to either wood or metal per shack. Many of the metal shack wall pieces have cool atomicpunk circular bits that will not mesh well with the right angles of the wooden pieces.