"Nothing so facilitates good writing as actually having something to say." T.S. Eliot
Source: "What Makes a Question Essential?" ASCD. ASCD, 2014. Web. 17 Nov. 2014.
Shakespeare is such an important writer that he has his own Dewey number. The works by Shakespeare can be found at 822.3. Critical analysis and writings about his work can be found at 822.33.
Try searching Shakespeare, William as a SUBJECT
Consider looking at the overviews and biographies at the beginning of your research. Reviewing what topics are covered in the literature criticism is also helpful - but skim for ideas at this point.
Again, look to see what topics are covered but try not to focus on any one critical essay or academic journal at this stage.