But even slavery is too broad a topic. The first database you use finds over 5,000 entries about slavery.
What is it about slavery that interests you? You could examine the topic from many different aspects. You could examine the economic impact on the United States; you could explore the lynchings which took place; you could look at the role of native Americans in helping escaped slaves, or you could examine the effects of slavery experiences on survivors or even their children.
Whatever area of a subject you choose, try to pick one that interests you. You are going to be making an investment of time and energy, and if you lose interest early in the project, it will be hard to complete.
Let's take a look at some of the factors for you to consider in order to carry out an effective search for material on this or any other topic.