It's in Java; I would really suggest doing some Java tutorials just to get a feel for the basics. If you haven't done any programming before it's going to be a bit rough.
The best place to start, if you're intent on doing something modding-wise, is to use the stuff in the base game as a reference. I.E. you might look at how a ship system is implemented, copy that, and work from there. There's also documentation in starsector-core/starfarer.api.zip, but that's not going to be helpful unless you're familiar with some basic Java concepts first. It's just a function reference so you know what's there, and light on actual explanations.