To select credible sources you need to apply evaluation criteria. Evaluation of sources is an important part of the research process. It is important to find a balance between credibility and relevancy. If you're using outdated, unreliable, or simply non-factual information and sources, your project (e.g. paper or speech) isn't going to meet your professor's expectations.
Sometimes you can tell if a resource is bad pretty quickly. For instance, would you use the National Enquirer as a credible resource? Most likely not. However, it could be a relevant source if your topic was about sensationalism in the media.
It may help to think of information the same way you think about food. Some foods are "healthier" than others. For your research and information "diet", you'll need healthy food, like scholarly articles or government reports. Use the sugar and empty calorie choices sparingly - i.e., unfounded claims, etc.
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