Any 3D scene in Photoshop starts with some basic elements. In this video, learn the essential ways to create 3D primitive objects within Photoshop CS6.
Raymond, constructing 3D objects is an exercise in geometry. Certain parts of objects can be described with primitives, others (such as the saucer of the Enterprise) are better described as solids of revolution. See Part 5 in my 3D series where I create a wine glass shape this way. Some items can't be described easily at all in this way. If you are set on creating very complex items, you'll want to look at a higher end 3D program that allows sculpting.
How do you make multiple parts of a 3d model such as for example a Starship enterprise, or a machines from Star Wars? The parts are not all straight or cylinder in appearance. How can they be made of multiple layers to a appear as one object?