The obvious place to start are the WordPress sites themselves.
For Authors
For using WordPress as an author I would recommend the support site:
http://en.support.wordpress.com/
I know this concerns the commercial, hosted, version of WordPress, but the functionality is more or less the same. There’s enough content here to get a beginner up and running with writing high quality content on a blog.
For Developers
For the more technically minded, the wordpress.org codex pages are fantastic:
Specifically this one for getting started:
http://codex.wordpress.org/Getting_Started_with_WordPress
Videos
There’s also wordpress.tv for those who want to watch videos:
Although a lot of the content here is not tutorial based.
Final comment
I know this is all content developed by WordPress, but if you’re just getting started, there’s no better place. The quality of the documentation provided by Automattic for WordPress is actually the main reason I switched to WordPress. I don’t know a better documented web based publishing system. Open source or not.
Once you’re a bit more clued up on things, sites like this one (wordpress.stackexchange.com) are very helpfull, as are the hundreds and thousands of blog posts that Google might return for a WordPress search.