The simplest way is probably to get it via this download link and install it: https://getcomposer.org/Composer-Setup.exe
If you want to install it via command line please dont forget to add composer to PATH, if you’re new to putting variables into PATH, it’s quite easy.
Read about it here: https://hendrikthurau.enterprises/tutorial-how-to-add-system-user-variables-add-to-path-in-environment/
The GIT project can be found here: https://github.com/composer/getcomposer.org/blob/master/web/installer
If you’re using linux you can install via command line.
php -r “unlink(‘composer-setup.php’);”