You do not need advanced Java knowledge to get into Minecraft modding, unless you're trying to do some really complicated stuff. If you already know a high-level programming language, transitioning to Java will be a breeze. I'd recommend learning the basic syntax, setup, and overall flow of Java. That's pretty much all you need to start.
I would definitely recommend learning modding for 1.12.2 for now. Forge for 1.13 is an incomplete nightmare right now, and 1.12.2 is still pretty relevant, both here on the forums and in the modded playerspace as well. Learning in a well-explored version will be easier, and also make the eventual transition to 1.13 easier as well.
You can do this by creating an EventHandler and then adjusting the drop event for the block/mob of your choice. That's just the general idea though, you'll need to learn a bit more first, and probably work your way up to that. I'm sure others here can help you out : ) .