Drupal uses an externally-maintained repository the https://packagist.drupal-composer.org but this repo is scheduled to be deprecated in January 2017 by then we will be using https://packages.drupal.org/8 repo URL.
It is assumed that composer is already installed in your system. Make sure your composer version is 1.0.0 or higher, to check execute:
If it is lower than version 1.0.0, update it by:
Change directory to your Drupal root path.