Step 1: Setting up your development environment
I strongly recommend using IntelliJ over Eclipse. Having used both, I've found IntelliJ much easier to use/get started with. In addition, almost all the modders in the official Discord use it and will be more easily able to help you if you do as well. As such, this tutorial will be explaining how to setup with IntelliJ.
You can find the free community edition here. Installation instructions are also on that page.
Do not clone or fork unless you really want to go through the hassle of changing git things later.
Unzip wherever you want to keep all your modding files. I highly recommend creating a dedicated folder for modding, as well as an individual folder within it for each mod you create.
When you first open IntelliJ (after setup) you should be greeted with a screen like this. The color scheme will depend on what you chose.
Choose open, and then find the file pom.xml wherever you put DefaultMod.
Open it. This will give you a popup saying that pom.xml is a project file. Would you like to open the project?
Choose Open as Project. After a brief period of importing, you should have something like this.
If so, you can move onto the next step.