Setup my existing Drupal website to work with PHP Composer

I have lots of websites created with Drupal 10 - they were built manually and not with Composer.

I now need to install PHP Composer so I can manage my website dependencies.

I have tried to run composer commands from Putty SSH but keep getting the error '/user/bin/env: 'php' : No such file or directory'.

I have also installed the Plesk extension PHP Composer - but when I go into this on my websites I cannot update or install dependencies as it says 'Looks like there's nothing here' for installs or updates - I believe I need a composer.json file, but I am not sure?

I have no idea what I am doing so I need somebody to setup Composer on my Plesk server so I can use the command line, I also would like you to make one of my sites 'Composerized' with a composer.json file and tell me how to do this so I can do the other sites myself.