If you are using composer, you do not create the autoloader but let composer do its job and create it for you.
The only thing you need to do is create the appropriate configuration on composer.json and execute composer dump-autoload.
E.g.:
{
"autoload": {
"psr-4": {"App\\": "src/"}
}
}
By doing the above, if you have a file structure like this
├── src/
│ ├── Controller/
│ ├── Model/
│ ├── View/
│ └── Kernel.php
├── public/
│ └── index.php
└── vendor/
After executing composer dump-autoload the autoloader will be generated on vendor/autoload.php.
All your classes should be nested inside the App namespace, and you should put only one class per file.
E.g.:
<?php /* src/Controller/Home.php */
namespace App\Controller;
class Home { /* implementation */ }
And you need only to include the autoloader in your entry-point script (e.g. index.php).
<?php
require '../vendor/autoload.php';
Which will allow you to simply load your classes directly from anywhere after this point, like this:
use App\Controller\Home;
$homeController = new Home();
This is explained at the docs, here.