Tips for selecting a topic:
1) Choose a general topic that interests you. You should start fairly broad, like "Civil War Battles" or "Women during the 19th Century." Be prepared to narrow that topic down. Ultimately your paper will have a very narrow focus, but identifying an era or general topic that interests you is a good place to start.
2) Begin some general research to start to narrow your focus. Are there specific people that interest you? Strategies? Social movements? Events?
3) Determine if there will be sufficient material and sources to support a research paper. Remember, you must have primary sources; you may not rely solely on secondary sources. Sometimes these primary sources are difficult to come by. For example, if you are interested in ancient history, there may be very few surviving documents. If you are researching an obscure or controversial event, it may be hard to get your hands on primary source material. If the primary sources are written in another language, translations will not always be available.
Don't tie yourself to a topic without a thorough evaluation of available sources. Your sources will ultimately determine the direction of your paper.