Please click on the question mark icon in the top-right corner to access our help page. Here, in the "Usage reference" section, you can read about Todoist's features in detail.
If you'll experience issues with a certain features or have any questions about it, please don't hesitate to contact us anytime.
Here is the detailed blog post that may also be helpful for you: https://blog.todoist.com/2014/06/10/a-comprehensive-guide-to-using-todoist/
There is also a YouTube page to see video tutorials. http://www.youtube.com/user/IstProductivity