The Publication Manual of the American Psychological Association Style (a.k.a. APA Style) is primarily used by researchers and students in the following fields:
Most instructors will tell you which citation style to use for their assignments. If your instructor doesn't have a preference, you should use the citation style most appropriate for your career or field.
The citation style you use should be based on the journal into which you are seeking publication. Guidelines for publication can be found on the journal's website.