That all depends on what you're interested in. Is there a particular time period or nation or such that you're interested in learning about?

Id say start with the history of religions

Pick a subject you're interested (on just about any topic under the sun) and read three books about it. You can do this at the library for free or at your local bookstore if you don't mind shelling out some coin. Figure out where they agree and where they disagree. Figure out what sources they used. Figure out which writer you agree with.

Repeat this six or seven times on one topic and you're basically there. Or, follow a tangent after those three books and widen your search and you'll quickly become a generalist (which is also cool).