Which comes first?

OK, I’m a bit confused; I’ll apologize if necessary, as this isnt necessarily a DRUPAL Installation question, but it is Drupal related, and IS an Installation question, so…

I’m attempting to install Drush. As I’ve looked at Google, I’m seeing some places that tell me to Install Composer, and then use Composer to install Drush; while other places are saying to install Drush first, and then use Drush to install Composer… OR Does it matter which is first?

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/1/feed