You can use the in-game vehicle editor to create layouts for parts and accessories on a vehicle and save and distribute the .pc file, but this only tells the game what parts to use, and cannot include custom parts. To create a mod with a custom vehicle or parts, you must have some 3D modeling knowledge, and be comfortable editing game files (JBeam, json etc). To start I would recommend reading through the Vehicle Creating intro tutorial here. EDIT: Actually the link is broken because of the new forums, I currently do not know the new link for the tutorial. It was in the wiki, which I can't seem to access right now.
The new forums made my edit show up as a new post rather than adding a section to my above post..... Grrrrrr........ So now I have a useless placeholder post here because I can't delete posts. Current forums: http://wiki.beamng.com/doku.php?id=wiki:welcome
