Is there a tutorial on how to make new meshes? I don't know how to.
It depends heavily from what tool you plan to use. Currently, Blender is a very known and used Open Source 3D modelling and rendering suite, with some good tutorials
teaching how to use it. There's a Python plugin for Blender to import and export from/for Orbiter (I'm using it to measure some meshes using a Mac, and it works!).
I was advised to use
Anima8tor and
MeshWizard to deal with Orbiter Meshes - but my Windows machine is too old to run them, as it appears - so I didn't test them and can't give an opinion about. (And it's easier to find tutorials for Blender, anyway).
But dealing with meshes is somewhat like programming - you can learn by yourself, but best results are commonly reached by taking lessons from someone.
You will need some background from Computer Graphics to understand what are you doing. I took formal classes for this, implemented my own 3D engine (twice! one of them for an ARM powered mobile before the OpenGL/ES chips were available), so perhaps I'm not the best reference for a novice. But I
found this interesting, I hope it helps.
Be advised that you *DON'T NEED* to understand *HOW IT WORKS*, all you need to know is *WHAT* can be done and *HOW* to do it. If math and coding are not your style, run from CG lectures from Computer Science classes.
Meshing is somewhat as sculpturing - but you shape things by using vertices and lines (visual representations of numeric data, to tell you the true), not stone or wood.