Start by visiting your local or school library.
Use the library's computer database to search your topic using boolean operators. Boolean operators are "and," "or," "not" and parenthesis that can be used to expand or narrow your search. Example: (George Washington) and (Revolutionary War) not (Carver)
Use the bibliographies of any helpful books, magazines or journals you find to gather even more sources.
Use your local or school library's electronic journal database to search your topic, again using boolean operators. Some helpful journals may be JSTOR, Project Muse and LexisNexis, to name a few.
Use the bibliographies of any helpful essays or articles you find to gather more sources.
Ask the librarian to assist you with microfilm resources if the history you are researching is local.
Ask the librarian about interlibrary loans if there are any sources you find that could be helpful but cannot be found at your local or school library.