When you are assigned to choose a social problem, your first step should always be to check very carefully what exactly your professor expects, since "social problem" can be described and defined in a variety of ways. For example:
Some professors do not want you to use the easy and obvious topics that can be found in collections such as CQ Researcher and Opposing Viewpoints, and might even mark you down for citing those materials rather than scholarly sources.
Some professors make it clear that they want the social problem to be something observable (like lead poisoning from paint) and not your argument about a political or legal cause (that is, not the regulations or lack of regulations that - you might argue - allow children to have access to lead paint).
Listen carefully in class and read the assignment closely!
Explore before you make a choice! The Library subscribes to many reference and news sources and article databases that address social problems. Exploring them might bring up a fresh topic instead of the same dreary set that your professor sees year after year.
Exploring for a social problem:
These collections of reference articles, books and news sources are excellent starting places when choosing a social problem. Once you have chosen the problem, switch over to scholarly databases to find research articles.
Social work encyclopedia with overviews and introductions for many topics.