Credible sources make your argument stronger and more reliable. Scholarly, peer reviewed sources are always credible. You can evaluate other sources for credibility by asking:
Who is writing?
Should be an expert: Master's degree or higher in their field
Any doubts about the credibility of a source you've found? Ask a librarian!
For some research topics, you may want to cite original documents, interviews, artwork, artifacts, novels, poetry, or music. These are primary sources, which were either created at the time of an event or created by a person who experienced an event. They offer a unique insight into a particular time or topic.