Installing Drush on Mac OS 10.13 (High Sierra)
Drush 9 only supports one install method. It requires that your Drupal
  8 site be built with Composer and Drush be listed as a dependency.




See http://docs.drush.org/en/master/install/

There it's recommended you start your Drupal 8 project with Composer. Then add Drush as local dependency to your project and use this local Drush to do things.

$ cd drupal
$ composer require drush/drush
$ vendor/bin/drush cr


Using the Composer template for Drupal projects Drush already is listed as a dependency.

$ cd ~/Sites
$ composer create-project drupal-composer/drupal-project:8.x-dev some-dir --stability dev --no-interaction
$ cd some-dir
$ cd web
$ ../vendor/bin drush si


Additionally you then can install the Drush Launcher which will replace the global drush command with a wrapper that always automatically finds and executes the local Drush in every directory.



If all of this is no option for you you might want to have a look at Having multiple Drush instances for different core versions where I show how you'd still be able to install a global Drush quite easily.