As per the documentation, you need to include the autoloader.
E.g., in your php file:
require __DIR__ . '/vendor/autoload.php';
But... typically your
vendor folder would be at the same level than your
html folder (the one where you are serving content).
- Project root --- vendor --- html ----- index.php
Then in your index.php you'd do:
require __DIR__ . '/../vendor/autoload.php';
It is nice that you trying to use composer and modules, but if I were you I'd try to read on a bit about php's autoloading features, so you understand what's going on, and how you can profit from them.