Purdue OWL website gives a great description why we need to evaluate resources:
"Evaluating sources means recognizing whether the information you read and include in your research is credible. Despite the large amount of information available, both in print and online, not all of it is valid, useful, or accurate. Evaluating sources of information that you might include in your writing is an important step in any research process.
When writing research papers, not only will you be searching for information, but you will be evaluating the sources for credibility. You have to decide where to look, how to recognize credible sources, and how to cross-check your information. Learning how to evaluate effectively is a skill you need both for your course papers and for your life." (Purdue OWL Introduction)
Check out the following links from Purdue to help you with what you need to look for when searching for sources: