Always start with the basics. You should spend time checking out the Watson help menu. There, you will find that you have access to the following sources of information:
Detailed topics under Docs (from the Welcome page, click on the Help icon and then on Docs)
Little Hints that appear in various places in Watson (from the Welcome page, click on the Help icon and then click on Show all hints)
Expert blogs, discussion forums, and sample data (from the Welcome page, click on the Help icon and then on Community)