What is the best way to learn Java and learn how to make mods for Minecraft?
I mean there are are lot of tutorial on YouTube with people making mods that just make them. I can just copy them, I don't understand the code fully.
Learning java is different for everyone. If you can sit down and read/comprehend a book, do that, if not there is a website that will teach you java interactively. Just Google java interactive lessons. But that will really only teach you you the basics, taking a class at a university is great and will be a lot better than doing it solo.
Making minecraft mods will really be research on what goes into the regular game and how you can start making your own mods. Maybe through YouTube tutorials or documentary blogs on how to do it, you can start to get some ideas.