Scholarly periodicals (journals and peer-reviewed journals) are written for and by people who work in academics: professors, researchers, undergraduate or graduate students. This type of article is best suited for your research because it is reliable and authoritative. Many of these are peer-reviewed.
Conference Proceedings are a great place to find cutting edge research - if the conference is scholarship-based (rather than trade-based), you can expect to find cutting-edge scholarly ideas. Search in scholarly periodicals for further articles about an idea or theory you find in conference proceedings.
Theses and Dissertations are written by students, and while you should not use their work, you can definitely mine them for citations and testing instruments.
Scholarly books can be written by one or more authors, or an edited book with each chapter written by a different author(s). Check out the author's credentials to be sure of their authority.
If you have a journal and you need to check if it is peer reviewed, use the Ulrich's Periodical Directory database.
Used with great thanks to the University of Oregon Libraries