First important thing to do:
Ask yourself very skeptically, what you want to make. Not just simply say "I want this". Think about the how it should be like, make pencil drawings, write small texts how the player should use it, what you like. I prefer the pencil version simply because I can take my idea sketchbook with me whereever I may roam and need no electricity or internet for it. I am sure, as Indie game developer, this is no new idea for you.
Then, when you have decided on your goal and have a clear vision of the (intermediate) result. You decide about your tools. It is a good idea to already think about your current skills and the skills that you want to have in the future. You can't become a master of all trades, and at least you won't become it with the first add-on. Consider the possibility of making errors along the way and learning often the hardest way.
You will need a mesh in any scenario, so lets start there. i am no mesher, so I can't make you a good recommendation there. I think Blender has become a good free alternative, many still prefer Anim8or, AC3D is also popular, despite it costing you a bit of money. If you already know modelling, sure no big deal for you.
When I need a mesh, I prefer to ask for help by people who are more skilled there. That is also a viable option. And I deeply respect people for having those skills because my own attempts at making meshes are embarrassing in comparison. Again, this is a path where having good pencil drawings helps. Good drawings and ideas can help you convince people that its a good plan that you are up to. But it must not be successful. Even if you think you have the best greatest idea of all times, it can happen that you are the only one who thinks so and nobody wants to help you. Then you should decide: Is it? If yes, you should make the mesh yourself if you have that much confidence. Even if it is a bad mesh.
For everything related to making Orbiter add-ons, you should also get some help about physics. Not just astrodynamics, that is enough for playing Orbiter. Learn basics of aerodynamics, mechanics, newtonians physics, you name it. Learn math. Orbiter is written by somebody who knows linear algebra so good, that he made it left-handed. (pun intended) if you can understand what a vector is and how a matrix multiplication works, you are on a good way. The concept of affine transformations (from linear algebra) is very important for understanding how animations work in orbiter.
Then, look at the behavior of your spacecraft. If it is just static with some thrusters, you need no programming at all, a configuration file is enough. This is much easier than any kind of programming, and a very important basic skill in making Orbiter add-on. Its never bad to solve as much as possible with a configuration file.
If you need animations, you can avoid C++ code by two alternatives: generic vessel modules (like spacecraft3.dll) or Lua scripts. I am no expert in both, so don't expect much help there from my end... but there are people who can do magic there.
And finally... yes, there is C++. Not sure how much C++ you have seen in your life. C++ allows you to do it all. If you can think it, you can find a way to code it. But C++ has a price. It takes a long time to learn and will drive you mad while you learn it. It takes really long to master C++. And again, it takes away sanity. Not because it is a crazy programming language. Or a crazy way to do add-ons. But because it gives you freedom and freedom means also the freedom to make your own life hell. And that is what you will learn the hard away in C++. C++ not just allows you to shoot into your own foot. If you shoot into your own foot with C++, your whole leg will be blown to Mars. That is the power of it. If you can't control the power, the power will turn against you.
If I now failed to scare you and you still think C++ is your future skill: Learn in small steps. Get your Visual Studio set up, learn compiling the examples in the samples folder of Orbiter. Then make your own small add-ons. Maybe even make no add-on. Just some C++ program to try something before you get into Orbiter and have a more complex environment than a simple console application.
