For most topics, a simple web search can provide enough background information to get you started, and Wikipedia and Google are usually the best tools to use for that. But even though Wikipedia and Google are easy and familiar to most of us, both have some special features you can use to get better results.
For a topic such a "Freedom of Religion in the United States," a quick search of Wikipedia will most likely take you to an article that gives enough background to get you started. It gives you information about the "First Amendment to the Constitution," the "Establishment Clause," and the "Free Exercise Clause," - all topics you can follow up on. A simple Google Search would most likely bring up this result as well.
Google Advanced search allows you to specify government (.gov), educational (.edu), or organization (.org) websites, and that can help you tailor your results if you want a particular perspective. In the example below, there's an explanation from the U.S. Courts why school-sponsored prayer in public schools is unconstitutional.