There is no such thing to install composer in magento.
By the look of your terminal I am assuming that you are using windows CLI.
You just need to install composer in windows. Here is setup for composer, after installation you will be able to run composer through shell
May be this will help you:
This is one of the way, if you have not installed the composer.
download composer.phar file
Place it in magento root directory, then run the command like:
php composer.phar YOUR_COMMAND
How to Install and Use Composer on Windows :-