In order to find useful materials on the authors, like Joseph Conrad, who wrote Heart of Darkness, you will be covering in your essay, you need to survey the existing scholarship on the topic. Simply entering the author's name and title into a regular Google search will bring up student cheat sites, which offer poor quality work; also, no professor will be impressed by citations from cheat sites. Instead, what you need to access are "peer reviewed scholarly journals".
The quickest way to do this is to use "Google Scholar" (link in references below); it has the merit of restricting citations to legitimate scholarship and it is easy to use, but the search interface brings up a great deal of irrelevant work and not always what you need (Google algorithms are optimized for consumer products, not scholarship).
The best way to search for references is to go into the online catalogue for your university, and find the MLA database under electronic resources. It has a complete bibliography, with links to full text, of scholarly woks on literary topics and you can search for works on your texts.. If you have difficulty accessing this, your university library has reference librarians, who are available in person in the library and via online chat, whose job is to help students with research. Since your tuition pays for such support services as librarians, you should take advantage of the free expert help they offer to students working on papers. Reference librarians are an extremely valuable and knowledgeable resource.