How to install drush properly
If you read the Drupal compatibility table in the docs, you'll see that the correct version for Drupal 7 is Drush 8. The installation instructions for Drush 8 are simple and clear, no Composer required - just download a phar file. 

PS! If you are planning on maintaining Drupal 8 sites (with Drush 9) too, I recommend getting Drush Launcher.