You should begin your introduction with a brief paragraph introducing the topic that will be discussed in your paper. You should follow this paragraph with summaries of journal articles that you have looked up in the library. Each summary should contain something about what the study was trying to investigate, how they investigated the topic, what the results were, and what the authors of the journal article concluded about their study. A theoretical paper or a review paper will summarize and discuss 8 to 10 journal articles. If in doubt, always ask your instructor.
Citations are a major part of the introduction.
Pages 168-174 of your APA manual will give you more information about citations. Also, look for examples given in class. I do not give page limits to papers that are written in my courses because I want you to write as many Writing a Paper 4 pages as are necessary to do a good job discussing the topic. The subjects subsection describes who participated in the experiment or study. The characteristics of the subjects are described, such as where they are from, why they participated, perhaps their age, sex, and ethnicity, if you have that information.