The fastest way to get started with a customised WordPress theme is to create a child theme.
Find a theme that you can use for the parent theme, something that would make a good base, look for functionality that you would like out of your site so you won’t need to add it later.
To create a child theme the minimum requirement is a new folder in the themes directory with a basic style.css file.
Theme Name: Shane Jenkins
Theme URI: http://shanejenkins.com.au
Author: Shane Jenkins
Author URI: http://shanejenkins.com.au
Description: Child theme for the twentyeleven theme
Once you have the style.css file in place you can activate your new theme and start manipulating the child theme. You will never need to touch the parent theme files. If you want to change anything make a copy of the file and insert it into your child theme directory and manipulate your own copy.