Here's a short video explaining the parts of a scholarly article and how to read it:
Anatomy of a Scholarly Article (4:47) (NC State University Libraries page)
Here's an article explaining how to read a journal article:
Art of Reading a Journal Article: Methodically and Effectively (Journal of Oral and Maxillofacial Pathology). See the section on Types of Journal Articles, which gives a good short description of the different types of medical journal articles, differentiating between primary and secondary sources.
Reading deeper: Here's a web site that has checklists for what to examine when evaluating research articles: