Create a folder inside themes folder in wordpress directory.
Add following mandatory files inside the theme.
Inside style.css at the top add following comments:
Theme Name: themenamehere
Theme URI: www.sample.com
Author: author details
Author URI: author website url
Description: description about theme
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Only mandatory field is Theme Name : add yoru theme name in small letters here(eg: blog). If you wish, you can include remaining fields also.
Now you can see the created theme inside themes in wordpress Appearance >> Themes section.
You can add themes image as screenshot.png file inside theme directory: