On the bottom of your Admin page you will find a link to the Learn WordPress.com blogging Tutorial prepared by Staff. http://learn.wordpress.com/
The support documentation is all found at the Support link http://en.support.wordpress.com which is also on the bottom of your Admin page.
Note: the numerous videos and articles on the interent apply to WordPress.org installs and not to WordPress.com free hosted blogs unless they clearly state WordPress.com and we experience bloggers on this forum who are confused because they have relied on tutorials that do not apply to their WordPress.com blogs.
Now here are my opinions:
1. If you are committed to blogging over the long term then I recommend purchasing a domain mapping upgrade ASAP http://en.support.wordpress.com/domain-mapping/
2. If you have a professional or business site where you are building your brand I recommend you purchase a No-Ads upgrade. http://en.support.wordpress.com/no-ads/
3. If you have a personal blog and you have CSS editing skills and you are willing to pay for a custom design upgrade every year then go for it, play and have fun. But do don't fool yourself into believing that a custom design upgrade is required to create a successful blog. When it comes to attracting and maintaining readers it is the quality of your content (original and unique content) and not the container it's displayed in that attracts and secures faithful readers/subscribers/commenters.
Note also that the CSS editing is theme specific and any CSS editing you did cannot be used if you move your blog content later to a WordPress.org install. The same goes for premium themes too - they are not transferable.