Where you begin your search for information depends on what you already know about the topic.
Source: Sathrum, R. (2011, May.) The Literature of Science. Retrieved from: http://library.humboldt.edu/infoservices/staff/rls/litscience.html
Primary Literature
The easiest way to think about Primary sources to think about literature that bears witness to an event -- that describes the event in a "first-hand" or "primary" way. In science, the primary literature is that literature that report the results of original scientific research, the key example being the research article. Some other examples include conference proceedings, technical reports, theses and dissertations.
In some disciplines, informal communication is sometimes referred to as "grey literature." Examples of grey literature include email communications, social media, unpublished conference papers, colloquia, etc. Grey literature is difficult to access and retrieve as it is not published in the traditional sense that journal literature is published. The best place to search for grey literature is the Web (which will also pickup those unpublished communications that have been deposited to institutional repositories by scholars.)
Formal (i.e., published) communication through scientific journals, research reports, published conference proceedings, etc. is most efficiently accessed through bibliographic databases or bibliographic indexes specific to this type of literature.
Secondary Literature
The best way to think about Secondary sources is to remember that they are providing a second-hand account of the event or research. The secondary literature draws on multiple other sources, both primary (e.g., research articles) and secondary (e.g., books) and synthesizes what is known about a topic or a particular aspect of the topic. Review articles published in journals, scholarly books (also known as monographs), and reference tools (such as handbooks, manuals, dictionaries, encyclopedias, and atlases) are the best examples of secondary sources.
Review articles (as opposed to research articles) are a mainstay of science research. These types of articles are indexed in the bibliographic journal databases or journal indexes and usually there will be a way to limit your search to these types of articles specifically.
Access to monographs and specific reference tools of interest are best achieved by searching the Library's catalogue.
NOTE: Reference tools (e.g., handbooks, manuals, dictionaries, encyclopedias, atlases, etc.) are considered by some disciplines to be tertiary literature sources. For our purposes, we will include them in the secondary tier of scientific literature.
Tertiary Literature (or Access Tools)
The Tertiary literature includes those tools used to index and access the primary and secondary literatures. Examples include the Library's catalogue and the journal databases/indexes we subscribe to.