first you need to create a folder in wp-content/themes/YourChildthemeName
Suppose we are creating a child theme for twentyFifteen theme.
then we can put our child theme name to twentyFifteen-child
The most important thing about child theme is consist of two files that is functions.php and style.css.
These both files are just copy of the parent theme.
The style.css consists of below code just copy and paste:
Theme Name: Twenty Fifteen Child
Theme URI: http://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.