first, you need to create a folder in wp-content/themes/YourChildthemeName
Suppose we are creating a child theme for the twentyFifteen theme.
then we can put our child theme name to twenty fifteen-child
The most important thing about child theme is consist of two files that are functions.php and style.css.
These both files are just a copy of the parent theme.
The style.css consists of below code just copy and paste:
Theme Name: Twenty Fifteen Child
Theme URI: https://developergang.com/twentyFifteen-child/
Description: Twenty Fifteen Child Theme
Author: Developer gang
Author URI: http://example.com
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
Text Domain: twentyFifteen-child
You can make this file blank.