Note: Since I have a Windows machine this tutorial will focus on that platform.
Java Development Kit
You will need the JDK to develop in Java. You can get the latest JDK at the Oracle website.
Modding
Creating a non-official addon for a released game.
How to Learn Minecraft Modding
Learning for the sake of learning is an exercise in futility. You will often end up losing focus and will never have achieved anything. You need to set a clear goal, and then figure out how to meet that goal.