You may install Composer locally, as part of your project, or globally as a system wide executable.

Locally

To install, run these commands in your terminal.

php -r "copy('<https://getcomposer.org/installer>', 'composer-setup.php');"
# to check the validity of the downloaded installer, check here against the SHA-384:
# <https://composer.github.io/pubkeys.html>
php composer-setup.php
php -r "unlink('composer-setup.php');"

This will download composer.phar (a PHP Archive file) to the current directory. Now you can run php composer.phar to use Composer, e.g.

php composer.phar install

Globally

To use Composer globally, place the composer.phar file to a directory that is part of your PATH

mv composer.phar /usr/local/bin/composer

Now you can use composer anywhere instead of php composer.phar, e.g.

composer install