You need to know how to code faces, which are like the sides of a 3D object. Also, you'll need to know how to place textures onto the faces. Both require some knowledge of coding, the understanding of the x, y, and z-coordinate planes and some math. This is all done in Notepad or Microsoft Excel (I prefer Notepad).
If you're ready to create some exteriors, then go to the openBVE website (which has changed to
http://trainsimframework.org/), click on the "Create Add-ons" tab and look at the Objects section at the left of the page.