It's the same way you make models for any other games, just that you have to keep a balanced polycount and have a gridlike polyflow for better deformation. Watch tutorials and practice to get used with modelling. Modelling a car can take weeks, it's not a thing you can do in 24 hrs, unless you plan to have a rushed work. Blender is the software I suggest. It's free and many people uses it.
You'll have to get knowledge of modeling first before you can put guilty models into game, to start though create something in Blender 3D (download that from the blender 3D website, Google it) and put it into game by exporting it to DAE format. It it will take a few weeks of learning maybe months to know how to import good models into beamNG