You need Composer to be installed on your local machine before executing any Composer commands. Please see Getting Started on getcomposer.org to install Composer itself. If you are allowed to install Composer globally, do so as there is no benefit of having one Composer installation per user/project.
Downloading Commerce module
After get composer installed, locale to root folder of Drupal site then run the command bellow:
That command will download commerce module and all require libraries and module modules (Address, Entity, State Machine, Inline Entity Form, Profile)..
Installing DrupalExp Nation Commerce module
Login to Drupal site, click on Extends menu item on Admin toolbar or navigate to http://yousite.com/admin/modules then enable DrupalExp Nation Commerce module. This action will install all require modules and create default config/views etc..