Before instaling COMPOSER make sure you have completed SERVER SETUP - Click here to know more.
STEP 1
Run below command from root directort to Install the composer globally.
curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer
See if it works run command composer you will get composer welcome screen.
Composer version 1.7.2 2018-08-16 16:57:12
To update composer run
composer self-update
Now install backend.