Also includes dependency injection

MY_MODULE.services.yml

services:
  my_module.cool_service:
    class: Drupal\my_module\Services\CoolService
    arguments: ['@logger.factory']

/src/Services/CoolService.php

<?php

namespace Drupal\my_module\Services;

use Drupal\Core\Logger\LoggerChannelFactoryInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * CoolService returns what you give it in an array.
 */
class CoolService {


  /**
   * Logger Factory.
   *
   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
   */
  protected $loggerFactory;


  /**
   * Constructs the Cool service.
   *
   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $loggerFactory
   *   A Guzzle client object.
   */
  public function __construct(LoggerChannelFactoryInterface $loggerFactory) {
    $this->loggerFactory = $loggerFactory->get('my_module');
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('logger.factory')
    );
  }

  public function doSomething($someData) {
    $output = [
      'my_data' => $someData;
    ];

    $this->loggerFactory->error('Lets log a message!');
    
    return $output;
  }


}

Calling the service

$coolService = \Drupal::service('my_module.cool_service');

$returnedData = $coolService->doSomething('My Data');

echo $returnedData;