File structures seem a lot different in Drupal 8, and I’m still getting the hang of them. This is an example of a simple module I made, so hopefully it can be a good starting point for you too!

my_module/my_module.info.yml

name: Module Name Here
type: module
description: Description of the module
core: 8.x

my_module/my_module.module

<?php 
//basically any code you want your module to do
//ex: any functions that need API calls or data that you call in any preprocessor functions
use Drupal\my_module\Controller\DefaultController as MyModule;
function my_module_get_data(){
  $data = MyModule::hello_world();
  return json_decode($data->getContent());
}

**Drupal 8 includes a way to make URLs via a routing.yml file.
my_module/my_module.routing.yml

#GENERAL
my_module.hello_world:
  path: '/my_module/hello_world'
  defaults:
    _controller: '\Drupal\my_module\Controller\DefaultController::hello_world'
  requirements:
    _permission: 'access content'

my_module/src/Controller/DefaultController.php

namespace Drupal\my_module\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;

class DefaultController extends ControllerBase{
  public static function hello_world(){
    return new JsonResponse(true);
  }
}

So in this example: now if you go to http://mywebsite.com/my_module/hello_world if will call the defaultcontroller in your module and print out JSON you can use in an ajax call.  You’ll also notice I can use it on the server side as well (my_module.module)

To me this was a lot easier or a process than Drupal 7 was. Happy Drupal Coding!