Thank you, Raztoki!
I'm certainly going to read the getting started guide. I hope it will give me enough guidance to accomplish what I want to do. I want to create a plugin for downloading from a website that needs a login. Do you think there is/are already freely downloadable plugin(s) for such a task and that only needs some tweaking to fit it into JDownloader? Also, right now I know nothing about Java, but I can of course try to learn the basics.
Edit: I've googled 'getting started guide site:jdownloader.org' and it landed me on the 'Getting started as a JD-Developer' page. Is that where I'm supposed to go? That I have to turn into a kind of JD developer? I also don't know where I can download all those plugins you mentioned.
It'll be a rather steep climb for me, I guess.