Never change any file or code within a theme. Nearly all theme get updated by their authors, and you will loose all your customizations. That said, twenty thirteen, and for that matter, all default themes that come shipped with WordPress are excellent basis to create your own child theme. Even when twenty thirteen gets updated (which it definitly will), you will not loose your customization.
To create a child theme is extremely easy. Just create a new folder in wp-content, call it what you like, create a stylesheet.css file inside the folder and add the following to your stylesheet
/*
Theme Name: Put the name of your theme here
Theme URI: http://example.com/Put-the-name-of-your-theme-here/
Description: Twenty Thirteen Child Theme
Author: John Doe
Author URI: http://example.com
Template: twentythirteen
Version: 1.0.0
Tags: light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
*/
@import url("../twentythirteen/style.css");
/* =Theme customization starts here
-------------------------------------------------------------- */
and your done
Go and read more on child themes and how to do customizations here