The answer to this depends, of course, on what sort of an assignment you are given. Depending on the assignment, you might want to use popular sources, scholarly sources, or even sources from your personal life.
For example, if you are asked to write a paper about teens’ attitudes about a given issue, you could use sources from your personal life. You could ask your friends about their thinking and that would be an acceptable resource.
As another example, you might be asked to analyze the way in which the media reports a given story. You would then need to use sources from the popular media. Imagine that you were trying to write about how the media portrays issues of global warming. You would need to look at newspapers and television newscasts, for example, as resources for this topic.
Finally, there are assignments for which you need scholarly sources. For example, you might be asked to write about the impact of minimum wage laws on unemployment. For this, you would need scholarly sources because those sources would have analyzed the subject in ways that more popular sources could not do.
Thus, good resources can vary widely depending on the type of assignment you have been given.