Roland Paris, an associate professor at the University of Ottawa, has a nice summary on how to read critically based on a 5 step CLEAR approach.
He provides a concise explanation why critical reading is important:
Critical reading supports critical thinking which helps reduce how much your opinions are influenced by bogus information.
Critical reading and thinking are not the same as negative thinking. Critical thinking is doing some analysis to determine if there is sufficient and valid evidence for claims being made. Negative thinking is assuming the worst possible outcomes without analysis of other possibilities.
Howard Rheingold and other noted educators are creating a world-class resource for teaching critical thinking and Internet literacies.
Snopes.com sheds some light on the truthfulness of common Internet rumours.