Personally I didn't have any knowledge in Java before I got into modding (although I knew how to use C# so that helped me understand processes). If you are want to create your own mod, your best bet is downloading some other faction mods and looking at how they work to understand what is going on. You can actually achieve quite a lot towards creating your own faction mod without comprehensive Java knowledge.
This tutorial shows how to set up an IDE (A fancy name for a text editor that lets you program more easily than using notepad), and
This tutorial shows you how to create your own basic ship that you can use in a faction.
This tutorial shows you around the structure of a mod's folders and how to create your own system in a new game.
Hopefully this can give you an orientation to what modding starsector involves