Create a custom Service

Create a Service on Drupal 8

1.  Create a custom module (here : mytest)

2. Create the service Class
Example: src/MTService.php

<?php
namespace Drupal\mytest;
class MTService {
  public function run() {
    return "This is a test Service";
  }
}

2. Create service file and add the class as service
Example : mytest.services.yml

services:
  mytest:
    class: Drupal\mytest\MTService

Now you can call your service from other modules