Step One: Brainstorming
The Encyclopedia of Social Work by ISBN: 0195310365Publication Date: 2010-11-03This source will give you a solid overview of a topic from a social work perspective. Try searching for "poverty," "mental health," or "financial assistance."
You should start by getting a "big picture" of your topic. Make sure you have a clear grasp of how your the American response to your topic has changed and evolved throughout American history.
More general sources that give you an overview and a timeline are good places to start.
You might start by checking CQ Researcher (a governement-sponsored research tool that puts out weekly briefings on current issues) to see if they have a good, recent issue relevant to your topic.
For example, if you search for "Mental Health Care policy" you will find an entry from May, 2013 which includes a timeline that goes back to the 1840s.