When researching sources, it is important to use information that is reliable and appropriate. Not all material you find will be labeled as "scholarly" or "peer-reviewed", thus it will be up to you to analyze the material for such elements as currency, relevance, authority, accuracy and purpose. One of the useful tools to use is the CRAAP test and it works for just about any source you find - books, journals, articles or websites.
Currency - how timely is the information? When was it published and has it been updated since then? If it is a website, are there multiple broken links? For the topic you are researching, is the information current or out-of-date?
Relevance - does the information answer your research question or add something new to your knowledge of the topic? Does it meet the stated requirements of your assignment? Is the information too technical or simplified for you to use?
Authority - who is responsible for the information? Are the author's credentials/contact information provided? Can you find information about the author from other reference sources or have they been cited by others in their field?
Accuracy - can the information be supported by evidence from others? Is the tone/language of the writing neutral and unbiased? Are there grammatical or spelling errors?
Purpose - is the information there to teach, inform, entertain, sell or persuade you? Do the author's make their intentions clear? Is the information factual, someone's opinion, or propaganda?
When evaluating the quality of the information you are using, it is helpful to be able to identify whether you are using a Primary or Secondary source. In doing so, you will be able to recognize whether the author is reporting their first-hand experience(s) or relying on the views of others. It is also important to be aware of the differences, as you professor may want you to utilize more primary sources and fewer secondary ones, and you should be aware of the differences in order to make the correct choice.
Primary Source - this is a first-person account by someone who experienced or witnessed an event. The document has not been previously published or interpreted by anyone else.
Secondary Source - this is a source that is one step removed from the (primary) original source. The author of a secondary source reexamines, interprets and forms conclusions based on the information that is conveyed in the primary source.