WordPress is the most popular content management system and you can find tons of tips and tricks online on how to make the most of it.
The WordPress Codex
In the WordPress Codex you will find good articles on several interesting topics:
- How to Use WordPress
- Working With Themes
- Write a Plugin
- Contribute to Development
Resource list of WPMU DEV
If you are looking for an exhaustive list that will take you through different topics and demonstrate interesting options for you, this is a good choice:
http://premium.wpmudev.org/blog/wordpress-career-boosting-resources/
You will find multiple good tutorials on Google and step-by-step how-tos on YouTube