Citing a source is like adding adding a friend on a social network. Just as people judge you by the people you associate with, they'll judge your research by the quality of the ideas you associate yourself with.
Who, what, when, where, why, how. Identify this information about every source you look at. The most reliable sources make it pretty easy to find this information. If you're unable to come up with good answers to each of these questions, you may want to look for a more reliable source.
Once you've evaluated a source and decided that it's reliable, ask whether it's relevant to your topic. The most reliable source is not helpful if it isn't relevant to your project.
Who created the information? Is there contact information available?
What is the reputation of the creator? Is the creator a reputable and reliable scholar or writer? Is
the creator an expert on the topic? Are qualifications clearly stated?
Is organizational affiliation or contact information given?
For whom was the information created, based on content, tone and style?
Does the audience have a bias or point-of-view that might effect the information?
What conclusions are presented? What premises/claims are presented?
Does the evidence support the premises/claims and conclusions?
Is the information that is provided complete?
How does this resource compare to other resources on the same topic?
Are facts and claims documented through foot/endnotes, bibliography or other references?
Are there factual or typographical errors, or inexplicable omissions, in the information?
Are there any biases in the information?
What does it contribute to the literature in the field?
What do the authors NOT say? Is anything major omitted?
How current is the information?
Has significant research been done since the item was published?
Has it been updated? If so, when?
Why was the item written, i.e. to information, convince, sell, entertain, etc.?
What hidden agenda might the author(s) have?
Where is the author employed? Are institutional affiliations listed?
Where does the money for the research come from?