I think this tooth-achingly bubbly tutorial is probably the most accessible way to get started:
They use Fusion 360, which is free software that is very very good, but it does have a bit of a learning curve to it. But it’s worth learning because it is CAD and CAM software all in one, and it’s really versatile… and free!
I’m not sure what the 3D file types are, as we do our 3D modeling within our software° and then go straight to generating the toolpath… the only thing that comes out is our g-code which we send to the X-controller via UGS.
° We use Vectric software, which is also very good for CNC, but unlike Fusion360, which is free, Vectric charge about a billion dollars for their software. And since we own it, I’ve never really developed my Fusion360 3D modeling skills.