Drush command not found after Drush completely installed
Once you have installed Drush, you need to ensure that you ~/vendor/bin directory is in your $PATH, as explained in the installation instructions (Note: this link no longer works)

Add export PATH="$HOME/.composer/vendor/bin:$PATH" to your ~/.bash_profile or ~/.bashrc, and then run source ~/.bash_profile (or ~/.bashrc).

I would recommend against installing Drush 9 via composer require at this time. Sticking with Drush 8 is probably best; if you want to use Drush 9, use a simpler installation method, such downloading the .phar, or clone the repository and run composer install in the directory Drush was cloned to.