I'm having a hard time understanding how to apply Materials to parts in a 3D model so that when they are rendered, they look like the material they are supposed to. For example, I have a model of a water treatment system that consists of SCH80 PCV pipe, stainless steel pipe, and painted (blue) steel tanks. Can someone explain how to apply materials to these objects to get a very realistic rendering?
 
Thanks

One way involves assigning the material to a layer.  Any object on the layer then is depicted with the material.  Layer names might be based upon the material for easier identification.