Peer-reviewed journal articles have gone through a special review process (peer review, naturally). This means scholars of a discipline have read an article carefully and determined that it makes a worthwhile contribution and was researched correctly. This is why instructors prefer you to use peer-reviewed articles -- it guarantees that your sources are good ones.
To find an article, search the Journal Locator for the journal title. Here's an example of where in the citation you'll find the title.
If we don't have access, you can request the article through interlibrary loan.
Other Useful Biology Databases