Making a mod for RoR is not very easy, there is currently no vehicle creator, everything is done from scratch.
The vehicles are actually simple, and all the software
tools needed to create them are free.
There is a lot of reading and learning to do to, even just to make a box on wheels.
All the information is in the Docs section. A good place to start is to learn about the
truck file format and all the links in it (vehicle concepts, etc)
Having a look at an existing .truck file may help you better understand the information on the Docs.
That being said, it's a lot to read, and even more to understand. I don't think you'll be able to find anyone to explain it to you any simpler or better than the Docs do.
If you have the time, patience and genuinely want to learn, it's doable. But if you get discouraged easily, I'm not sure if it's worth your time
