First, check if Composer is already installed:
In a command prompt, enter any of the following commands:
composer list --help
If command help displays, Composer is already installed.
If an error displays, use the following steps to install Composer.
To install Composer:
Change to or create an empty directory on your Magento server.
Enter the following commands:
curl -sS https://getcomposer.org/installer | php mv composer.phar /usr/local/bin/composer
For additional installation options, see the Composer installation documentation.