Your question is pretty vague... could you elaborate? Are you wanting to make a mod? are you wanting to spawn in a vehicle? are you wanting to export an automation car to beam?
Im looking to make a personal vehicle that i designed that i want only to use myself. so basically i want to make a mod but only i want to use it. And i would like to know how to make a mod in beamng. (because i cant afford automation)
You should just save the $30USD for automation, wait for it to go on sale if you really have to. The wiki (which is planned to be replaced) has a page on modding but creating a vehicle is far from easy. If you were to pay yourself minimum wage for creating a vehicle, assuming you already know how to do it, it wouldn't even be a contest between which is cheaper. BeamNG is a complicated game, which means it's complicated to mod. Even on the forums there's only a handful of users capable of creating a mod of quality worth using. Even if you wanted a bear minimum low quality mod that most people would not consider using then you'd have to get a hold of a 3d model somehow and slap it onto an existing jbeam. If you insist on learning how to make a quality mod from scratch, start off with easier things like skins and license plates. Then work your way up to parts for cars. And so on.