This is a great series for beginners, teaching the fundamental concepts of WordPress development so you can take the leap from tinkerer to developer. By the end of the series, you will be able to create your own rudimentary themes and plugins, and flesh them out with your own features.
When you are developing a WordPress Theme you can download this Test Data. The Test Data includes images and examples of every type of content that a WordPress blog might contain. Check that your Theme is looking good in all circumstances by Importing this Data into WordPress by going to Tools > Import > WordPress
The easiest and quickest way to install a WordPress test environment to your local machine is to download Instant WordPress.
You can use Instant WordPress to test out your WordPress Template without the need to upload to an external host. Instant WordPress comes complete with it’s own built in Apache web server, PHP and MySQL installations that are started and stopped automatically.