On behalf of the developers, contributors and user community: welcome to ROOT!
The ROOT beginners’ guide (aka “Primer”) is certainly the first document to read to master the power of ROOT.
The Manual introduces you to the main parts of ROOT.
After this introduction, tutorial presentations (and video!) offer a different, more direct approach to the ROOT fundamental concepts as well as some hands-on exercise. In order to integrate the information present in the aforementioned sources, this collection of ROOT courses is definitively useful. Once the fundamental concepts have been acquired, a rich set of tutorials are available to dive into the code.
Remember, you can always resort to the ROOT Forum to find out if someone already solved the problem you are facing or to get help!