What do I need to learn about Java to make a Minecraft mod (one with self-created creatures)? How to integrate code and textures?

Creature behavior is written as .java and compiled into .class with jdk, material those things in the texture folder, specifically you can find a mod, change the suffix to .zip decompression to see the specific categories. As for the .java how to write you can decompile minecraft, there is a format (forge API seems to have been decompiled)