The Beginner's Guide to WordPress Theme Development | http://www.ibenic.com/beginners-guide-wordpress-theme-development

child.php

<?php

add_action( 'wp_enqueue_scripts', 'child_theme_styles' );

function child_theme_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
 
}

dynamic_sidebar.php

<?php

if ( is_active_sidebar( 'main-sidebar' ) ) : ?>
    <ul id="sidebar">
        <?php dynamic_sidebar( 'main-sidebar' ); ?>
    </ul>
<?php endif; ?>

enqueue.php

<?php

add_action( 'wp_enqueue_scripts', 'enqueue' );

function enqueue() {
  
  wp_enqueue_style( 'style', get_stylesheet_uri() );
  // Example of another CSS: inside the theme folder /css/ without any dependencies (false), version: 1.1 and for all media
  wp_enqueue_style( 'style-css-2', get_template_directory_uri() . '/css/style-2.css', false, '1.1', 'all' );
  
  // Enqueueing Script inside the theme folder /js/ with jQuery dependency. Version 1.1 and enqueued in the footer (true)
  wp_enqueue_script( 'script', get_template_directory_uri() . '/js/script.js', array ( 'jquery' ), 1.1, true);
}

functions.php

<?php

function my_theme_function() {
  
  register_nav_menus( array(
        'primary'   => __( 'Primary Menu', 'myfirsttheme' ),
        'secondary' => __('Secondary Menu', 'myfirsttheme' )
   ));
}

add_action( 'after_setup_theme', 'my_theme_function' );

get_sidebar.php

<?php

get_sidebar( 'main' );

loop.php

<?php

  if( have_posts() ) {
    while( have_posts() ) {
      the_post(); // set the next post in the loop as the current post
      
      //Display the Title
      the_title();
    }
  }

sidebar.php

<?php

add_action( 'widgets_init', 'theme_widget' );

function theme_widget() {
    register_sidebar( array(
        'name' => __( 'Main Sidebar', 'yourtextdomain' ),
        'id' => 'main-sidebar',
        'description' => __( 'Sidebar Description', 'yourtextdomain' ),
        'before_widget' => '<li id="%1$s" class="widget %2$s">',
	'after_widget'  => '</li>',
	'before_title'  => '<h2 class="widgettitle">',
	'after_title'   => '</h2>',
    ));
}

style-child.css

/*
Theme Name: Your Theme Child
Theme URI: https://your.theme.url
Template: yourthemename
...
*/

style.css

/*
Theme Name: Your Theme Name
Theme URI: https://your.theme.url
Author: Theme Author
Author URI: Authour URL
Description: Theme Description
Version: 1.0
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Text Domain: yourtextdomain
Tags: blue, white, responsive
*/