This is a good place to start but if you want to do modelling stuff start but just making a simple part for an existing car for example a roof scoop or roof spoiler as those don't need too much or any jbeam you can just group them to the body node group usually.
Yes The easiest part to make would be something in interior, because it only needs 3d model, textures and dont need jbeam
hood scoops and spoilers don't even really need textures to look good, so that's even easier. interior elements still need jbeam editing.
this is a good tutorial if you want to make simple things like bodykits. covers pretty much everything except how to edit the jbeam physics structure.