The sources you use lend you the author's credibility and understanding of the subject--so find a knowledgeable expert! Research articles also provide you with evidence for the statements you are trying to make in your own writing.
When using scholarly, peer-reviewed journal articles, the emphasis of evaluation is on the content and currency. A simple way to evaluate any information is to consider its credibility, content, and currency (the 3 C's). Consider:
Content of the article
Does it attempt to answer your research question? Does it tell you something new?
Will it be interesting for you to read?
If you could have anything, is this exactly what you were looking for?
Currency of the article
Was the article published recently? Does it reflect the current research and findings on the subject?
When considering currency of scholarly journal articles, remember they are typically published quarterly (4 times/year) and it can be one year or more between the time the research is conducted and an article about it is published.
Credibility of the author or publication in which the article appears
Who wrote the article? What makes them an authority or expert on the subject?
Google the author to find out what makes him or her an expert. Advanced degree(s) in the subject area? History of publications on the topic? Employed by a reputable university/college or educational/research organization?
What is the type/reputation of the source?
Read the "About" section for a journal, magazine, or other website to learn about the scope and mission of the publication. For example, read the description of Developmental Psychology,published by the American Psychological Association.
For more about different types of publications, see this chart describing scholarly, popular, and trade publications.