Not sure what you want to investigate yet? Browsing the shelves can be a good way to find that first spark of inspiration. Below are the general history call numbers to get you started. However keep in mind that history can be a highly interdisciplinary subject, so once you get started with the research process don't worry if your work takes you away from these numbers.
If this is your first time using the Library of Congress call numbers don't hesitate to ask a librarian for help.