Trouble is, it's an SDK... If you don't know already what that is and how to use it in general, you have a long way to go.
In short, it's a library that lets you interface your own C++ code with Orbiter. It's not an editor or anything. It's code. You don't "use" it, You program against it. That implies knowing how to program. in which case, you would probably already know what an SDK is and how it works, so going on your question, I'll take the risk and assume that you don't know programing. Which means if you want to learn to use the Orbiter SDK, you best start with general programming.
However, your stated goal of "I want to make a white tank" doesn't really require any code.
In fact, depending on what you mean by "tank" it might only require a .cfg file. If by "tank" you should not mean "big can containing liquid stuff", but instead "heavily armored vehicle", it gets a bit more complicated, though
generalVehicle might be a good option.