On behalf of the developers, contributors and user community: welcome to ROOT!
In order to start mastering the power of ROOT, the first document to read is certainly the ROOT primer. After this introduction, these slides (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, these resources are definitively useful. Once the fundamental concepts have been acquired, a rich set of code examples can be found here.
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!