I'de start at wiki and just run down its links of sources. Google Scholar is a good place that I use for most of my college papers just to get a nice start. It also allows you to narrow your search easily down to .org or .gov websites if your teacher is pissy about .com's (mine was).
Once you find some things to influence the paper, you'll find there is always another link around the corner.
But yeah, I'de imagine your going to start with the whole Genocide of the Indians, and Slave trades, as those are pretty big ones.