Make a list of 25 topics that interest you, and be specific. Don't just list history as a topic, but rather list specific historical events or periods. Rank the topics in the order of interest you have for each one. Narrow the list down to your top 10.
Show the list to at least 10 friends or family members. They should be people you trust. You want to get an honest opinion. Have each person rank your top 10. Average out the scores and narrow the list to the top five choices.
Go to a bookstore or use the bookstores's online database to find other books which deal with the top five topics. Some subjects have been written about so much that it can be difficult to bring a new spin on the idea. Cross out the overused topics or get even more specific to make it unique. For example, pirates have been written about by countless scholars and authors, but only a handful have written about Benjamin Franklin's connection to pirates during America's conflict with Britain. Narrow the list to the top three choices.
Write out outlines for the top three topics. This lets you see whether the topic has enough information of interest to fill an entire book. A true story might not have enough events to justify an entire book, but it might if you decide to make it historical fiction. If all three topics have enough for an entire book, rank them to see which one holds your interest most.