It's a bit difficult to explain. If you are interested in programming and want to learn that, I recommend you to start with Java and blueJ tutorial. It's really good and teach you the basics. It is a nice tutorial for absolute beginners. https://bluej.org
About modding: it's a bit easier, you won't need programming skills, it's more like Scripting. If David finish that, there will be a manual for the api and you have to read that, that way you'll get what to do.
Ps: but I recommend you to try the blueJ for basic skills