When we categorize sources as primary, secondary, or tertiary, we are basing this on their proximity to the origin. These categories will change depending on our research topic and discipline. Different source types are used for different purposes.
The important thing to remember is that:
Primary sources are original materials that you will explain, analyze, or interpret. They haven't been evaluated or interpreted by anyone else. They look different depending on your discipline and topic. In STEM fields, primary sources may be research articles describing the design and findings of studies conducted by scientists or patents. In the humanities, primary sources may be works of art or literature. In social sciences, primary sources may be journals or letters of people alive during a historical event, government documents, interviews, field research, or newspaper articles.
Any source can be a primary source, depending on the context and your research question. If you're writing a research paper about recent winners of the Nobel Prize in Physics, the Wikipedia page for physicist Donna Strickland would be a tertiary source (an not an inherently reliable one). If you're writing a paper about gender bias on Wikipedia, and you learn that a draft page on her had been rejected for not demonstrating significant coverage until she won the Nobel, the same site would be a valuable primary source.
Since any source can be a primary source, depending on your research question, there's no one simple way to search for them. You'll have to figure out what primary sources for your topic will consist of. Consult with a librarian or your instructor if you're having trouble.
Secondary sources utilize primary sources as evidence to describe, analyze, explain, or interpret these primary sources. These sources provide valuable context about primary sources. Most scholarly books and articles are secondary sources. The important feature of secondary literature is that they utilize primary sources as evidence. Secondary sources may be books, articles, histories, biographies, literary criticism, legal commentary, political analyses, or other forms of criticism, commentary, or analyses. Secondary sources are primarily produced by and for experts and academics, not the general public, though there are exceptions.
Oftentimes, the secondary literature is referred to as a conversation among scholars across time. It is important to know the highlights of what has been said in this conversation on your topic, in order to understand where your unique contribution fits in. By engaging in academic research and writing, you are joining in and contributing to this conversation.
Tertiary sources index, organize, compile, digest, or summarize other sources. Many reference materials and textbooks are tertiary sources. The primary use of tertiary sources in academic research is for background information (for example, to help you form a list of searching keywords) and to point you to important primary and secondary sources via footnotes, source lists, bibliographies, etc. Generally speaking, tertiary sources are not to be cited in academic research.
A high quality tertiary source can give you a valuable birds-eye view of a topic and help you quickly find key authors, texts, perspectives, dates, and other information and sources. If you're unsure whether the tertiary source you're consulting is high quality, cross check the information you find with several other sources, or you can consult a librarian or your instructor.