Before we install Drush, we need to install globally Composer. Composer is a PHP package manager akin to NPM or Bower.
Next we want to edit our .bash_profile. Go your home folder
Create a new .bash_profile, (don't worry, if you have one, this won't overwrite it). We need to add a global entry for Composer.
Add the following to your .bash_profile
Now that we have composer installed globally, we can install Drush via composer.
Finally, we can select a specific version. For Drupal 8, we want Drush 8.