How would I create a thesis statement on "Why is William Shakespeare the most important figure in English literature?" and an outline of the essay that supports that thesis?

You thesis statement should read something like this: William Shakespeare is one of the most important figures in English literature because of his contribution to the language itself, his influence on drama and writing as a pastime, and the volume of literature he produced.
From this thesis, you can build the essay based on the three separate areas that are mentioned. Shakespeare contributed a great number of actual words to the English language, such as "alligator." These contributions have changed the English language since his time, and he perhaps made more than any other single individual in history.
The next section would deal with his influence on drama and writing as a pastime, as he made theater more accessible to the masses with his plays and with the Globe Theatre.
The final section would discuss the volume of literature he produced, including his hundreds of sonnets. The sheer number of productions he created ranks him highly for English language figures and, coupled with his other contributions, easily makes his case for the single most influential contributor to the English language in history.