You'll want to assign and attach materials to the mesh so that it appears in-game. The model is just a huge number of invisible points in space. The texture/material is what makes it visible.
You also want to have a read at our wiki. https://wiki.beamng.com/Introduction_to_Vehicle_Creation And perhaps start with something smaller, making a vehicles is hard, complex and requires time. You want first to practice with easier things. Some 3D modelling knowledge is also very good to have (basic stuff, like what mesh origins are, and so on)
