The easiest way is using an existing theme and copy the whole directory. A good start to read about creating themes can be found at http://codex.wordpress.org/Theme_Development
WordPress Theme Description
The theme information itself is part of style.css and can be found at the top of the file. Let’s create a new one based on Twentythirteen.
Modifying the style.css:
Theme Name: Clean 2014 Theme URI: http://wordpress.org/themes/clean2014 Author: RR Author URI: http://wordpress.org/ Description: The 2014 theme for WordPress creates a simple theme based on WordPress Twentythirteen Version: 0.9 License: GNU General Public License v2 or later License URI: http://www.gnu.org/licenses/gpl-2.0.html Tags: black, brown, orange, tan, white, yellow, light, one-column, two-columns, right-sidebar, flexible-width, custom-header, custom-menu, editor-style, featured-images, microformats, post-formats, rtl-language-support, sticky-post, translation-ready Text Domain: clean2014