Download, download

Now that you are all set with the new a shiny, new macOS. Let's get downloading!

Step 1: Xcode

Xcode is an integrated development environment for macOS containing a suite of software development tools developed by Apple for developing software for macOS, iOS, watchOS and tvOS.

Download and install it from the App Store or from Apple's website.

You can simply go to App Store > Click on search at the top right > Type Xcode > Look for Xcode (Developer tools) > Click and proceed with the installation via Install button.

You can also run:

$ xcode-select --install

This will allow you to install both Xcode and Xcode command line tools (which btw is not a neccessity).

Step 2: XQuartz (Optional)

XQuartz is Apple's version of the X server, a component of the X Window System for macOS. It is useful for developing software for the macOS, it's available for download here. Also, its free.