WordPress is the most popular CMS on the web. Opensource, written in PHP, and connecting to a MySQL database it uses a technology stack that is almost universally installed. I find that its ubiquity is at once a source of its greatest strengths: a large pool of developers to hire from, exhaustively detailed documentation; but also of its most frustrating short comings: it can be the target of hacker attacks, its theme and plugin marketplaces are polluted with nonsense.
To get started download and install MAMP on your computer.
Flat File CMSs
While some developers have devoted their careers to WordPress themes, there are an innumerable amount of CMSs out there. We will look at Kirby and Grav as interesting examples of CMSs that simplify development by omitting databases.
Install WordPress locally on your machine, and develop a small theme with just the basic pages for functionality.