You can't just convert a sheet or a 2D plan into 3D. It has to be modeled utilizing the 3D modeling tools. If you do have a plan of something like a house you can extrude some entities. (i.e. a 2x4 stud in plan can be extruded into a 3D surface with a certain height to resemble the actual stud.)
UCS = User Coordinate System and WCS = World Coordinate System. Do a search in the help file (F1) to get more info.