Remember that no one expects you to be an expert in everything. You are, therefore, expected to give credit where credit is due. Citing sources makes you look more knowledgeable and professional. When in doubt, cite it.
Still not sure? Take 10 minutes to complete this tutorial about citation and plagiarism created by librarians at Vaughan Memorial Library at Acadia University.
APA Formatting Guidance - Reasearch Papers
APA Formatting Guidance - Annotated Bibliographies