How to create simple module in magento 2

Let's name the module StackExchange_HelloWorld.
you will need these files:

app/code/StackExchange/HelloWorld/registration.php - the registration file

<?php 
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'StackExchange_HelloWorld',
    __DIR__
);

app/code/StackExchange/HelloWorld/etc/module.xml - the module declaration file

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="StackExchange_HelloWorld" setup_version="2.0.0" />
</config>

app/code/StackExchange/HelloWorld/etc/frontend/routes.xml - the frontend routing file

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:App/etc/routes.xsd">
    <router id="standard">
        <route id="helloworld" frontName="helloworld">
            <module name="StackExchange_HelloWorld" />
        </route>
    </router>
</config>

app/code/StackExchange/HelloWorld/Controller/Index/Index.php - the index controller

<?php 
namespace StackExchange\HelloWorld\Controller\Index;

use Magento\Framework\App\Action\Context;
use Magento\Framework\View\Result\PageFactory;

class Index extends \Magento\Framework\App\Action\Action
{
     protected $resultPageFactory;
     public function __construct(
        Context $context,
        PageFactory $resultPageFactory
    )
    {
        parent::__construct($context);
        $this->resultPageFactory = $resultPageFactory;
    }
    public function execute()
    {
         $resultPage = $this->resultPageFactory->create();
         $resultPage->getConfig()->getTitle()->set(__('Hello World'));
         return $resultPage;
    }
}

app/code/StackExchange/HelloWorld/view/frontend/layout/helloworld_index_index.xml - the layout file

<?xml version="1.0"?>
<page layout="2columns-left" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceContainer name="content">
            <block class="Magento\Framework\View\Element\Template" template="StackExchange_HelloWorld::index.phtml" />
        </referenceContainer>
    </body>
</page>

app/code/StackExchange/HelloWorld/view/frontend/templates/index.phtml - the template for the block

 <h2>Hello World</h2>

after you are done, run this in the console

php bin/magento setup:upgrade

You should be able to see the result at the url [ROOT]/helloworld


Well this is a broad question but my best advice would be to check the official Magento 2 samples.

You can find them here: https://github.com/magento/magento2-samples

This project is a collection of samples to demonstrate technologies introduced in Magento 2. You will find the most simple extension along with samples that incrementally add features to lead you through a exploration and education of the Magento 2 platform.

On top of that you can find many tutorials if you search "magento 2 create module" in Google