Creating a plugin for WordPress is easy. It will only become harder if you want to make something complicated. Adding content to article body, adding some scripts or styles are some easy cases.
A plugin is a folder which contains a php file at bare minimum. It will be placed under wp-content/plugins/
for WordPress to recognize it.
Firstly, create a folder called first_plugin under wp-content/plugins/.

Secondly, create first_plugin.php in the created folder.
The file’s content:
<?php
/*
Plugin Name: First Plugin
Plugin URI: https://www.www.tldevtech.com/
Description: Just our test
Author: TL Templates
Version: 1.0
Author URI: https://www.www.tldevtech.com/
*/
Now, you can add some codes to this file to do something. Try adding content to article’s body.