You can begin your search at your school library or on the library's website. When searching the library's databases, you will want to use a variety of keywords. First try "Langston Hughes' [and] poetry" as your set of key words. These key words will give you a broad expanse of sources (books and articles) that address Langston Hughes' poetry. You will have to narrow your search by locating sources that address the poem "Let America Be America Again." You can also try the search term "Let America Be America Again." This search term will provide a narrow list of sources that only address this poem. Alternatively, try both search terms together.
You can also use these search terms on Google Scholar: scholar.google.com. You can adjust your search in Google Scholar to provide peer-reviewed articles or links to books on Google Books. Additionally, use the "settings" feature to add a university library.
You can also try the Beinecke Rare Book and Manuscript Library, a subset of Yale University Library's website, which features a section titled "Langston Hughes at 100."
First of all, because Langston Hughes was such an important voice for black people, you will find many books about him and about his poetry. You will definitely be able to find a book about the poem you have chosen. Of course, the internet will have several sources you can check about this poem as well. Enotes also should have information about Langston Hughes' poetry which should also help. The second part of this assignment must be about what the sources had to say. Are you looking for the theme? Are you looking for whether this poem was influential in the time it was written? Think about not only the sources and where to find them, but also for what you are looking. Hughes is a giant among the black writers in American Literature. Good luck.