The answer for you is very long ... too long for this forum.
Start off with going to the main Orbiter Forums at orbitersim.com. Go to the Addon Forum and read the stickies at the top of
the page. This is just a starting point. I would highly recommend you visit Harmsway's site.
In a nutshell, you build you model in animator. Convert it to a 3ds file. Convert the 3ds file to an Orbiter mesh file with
max2msh.
Have fun ... modeling in Orbiter can be quite a career.