probably start off by learning the software, learning how to use blender would probably be a good first step.
well i do know how to use blender, infact i made a model of a mitsubishi lancer that doesn't look so bad (btw the thread link is https://www.beamng.com/threads/2004-mitsubishi-lancer-or-hirochi-carisma.108907/), but the issue is bringing the car itself to beamng
as others did say, starting small would help you in the long run. gain skills and build up to the full car mod, instead of throwing yourself in the deep end.