I have to write an essay about Ibsen ,but I can not find enough information about him ,especially about his writing style and his creative background .
Googling key words in various combinations can be a great way to generate ideas and information (Ibsen writing style creative background, for example).
Famous persons often have organizations devoted to their study, too. Have you tried the Ibsen Society? Their website has a wealth of information, including a biography of his life with comments about his writings, and annotated bibliographies of scads of articles about Ibsen.