There are three different versions of Drupal that you could download and install today: Drupal 6, Drupal 7 and Drupal 8. On top of that, maybe you’ve heard about this Drupal 9 that’s coming out. Which one should you choose? What’s the difference?
First off, Drupal 6, while popular, is no longer being supported by the Drupal community. I would not build a site in Drupal 6, especially since there’s no benefit these days to doing so.
Drupal 7 is the most popular version of Drupal right now and will be supported until November of 2021. Drupal 7 has a lot of things going for it: extensive documentation, lots of public modules, and plenty of support from those who have websites on it. However, it is super difficult to migrate a Drupal 7 site to Drupal 8, so upgrades after November 2021 will be much harder and the site will likely have security issues. In 2019, I don’t recommend building a Drupal 7 site, especially since it will teach you outdated ways of using Drupal and it would be better to start with Drupal 8.
Drupal 8 has less documentation and public modules, but it is currently supported. In addition, when Drupal 9 launches in June 2020, the migration will be as simple as a core update. This means creating a Drupal 8 site may be more difficult up front, but in the long run will be much easier to maintain. As we go forward, I’m going to focus on creating a Drupal 8 site since I believe that is the best bet for building a Drupal site in 2019. If you want to build a Drupal 7 site, then good news! There’s lots of options on the web for learning how to do that. I’d start here: https://www.drupal.org/docs/7/install