When your instructor tells you to use journal articles and not preprints or conference papers, library databases have labels and filtering to help. See the examples below. If you'd like to know more about primary (or secondary or tertiary) sources, staff of the Virginia Commonwealth University Libraries have a guide, Identify Primary, Secondary, Tertiary Sources.
This example is from Compendex:
In this example from Inspec, notice that you may filter the results by document type; for example, ask to see only journal articles.
Academic Search Complete indexes few conference papers or preprints but does index some popular material. You may limit to find or exclude those items and to select for peer-reviewed articles.
"Journal article" usually means "peer reviewed" but there are a few exceptions. Journals may include editorials and news updates. To check on your own to see if a journal uses peer review:
Authors and readers like the idea of rankings and assigning value to journals. There is a product that does that:
Read more about the topic here:
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