TYPO3 v9 has reached its end-of-life September 30th, 2021 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
To follow along with this documentation you need to have:
The system extensions Extbase and Fluid installed.
The Extension Builder extension installed.
The Extension Builder helps you to create and manage your Extbase and Fluid based extensions and comes with a backend administrative module.
Note that you should still familiarize yourself with the files that the Extension Builder generates and their usage.