Composer

Composer can hadle CVS (git, svn ...) and ZIP packages.
To install/Update your custom modules, add a custom repository to the composer.json file, following this example.

1. Add package repositories
Here, youtubeapi and sadb

Get drupal8 using default Packagist.org (The composer packages repository) drupal repository.
composer create-project drupal/drupal drupal8

Install Composer

wget https://getcomposer.org/installer
 php installer
 unlink installer
sudo mv composer.phar /usr/local/bin/composer.phar
alias composer='/usr/local/bin/composer.phar'

Run : which composer

Result exemple : /usr/local/bin/composer