Once you have a general idea of what's at stake in your topic (context, major players, historical significance, etc.), you'll be ready to look at it in a little more depth. Books (in print or digital) will give you depth and breadth.
When historians talk about evidence, they often mean primary sources. Primary sources (diaries, letters, speeches, newspaper accounts, artifacts, etc. created during the time period being studied) can offer rich and unique insights into your topic. Thousands of primary documents have been digitized and can be found online on university, government and museum websites. The Bentley Library also has collections of these documents in print and ebook sources. Check out the "primary sources" research guide and/or use the custom search engine below.