Here's a great definition of primary sources  from the American Library Association:

"Primary sources are original records created at the time historical events occurred or well after events in the form of memoirs and oral histories. Primary sources may include letters, manuscripts, diaries, journals, newspapers, speeches, interviews, memoirs, documents produced by government agencies such as Congress or the Office of the President, photographs, audio recordings, moving pictures or video recordings, research data, and objects or artifacts such as works of art or ancient roads, buildings, tools, and weapons. These sources serve as the raw material to interpret the past, and when they are used along with previous interpretations by historians, they provide the resources necessary for historical research."

--Using Primary Sources on the Web, rev. 2008.

Start with the basic questions:

·         What? – What type of material is it? Document, photograph, government publication…

·         Who? – Who created the material? Where did it come from? Can its origin be determined?

·         When? – Where does the item fit into the chronology of the period being studied? How close to the person or event is the material?

·         Why? – For whom and to what purpose was the material created? What biases may inherently or intentionally exist in it?

Interpret your findings—given the facts about the items, interrogate it:

• What content is provided and what is missing?
• What items do you need to create as objective of a study as possible? Where does the material point you now?
• What verification or additional research is needed to flesh out your understanding of the material? Additional secondary sources? Different types of primary material?

Interrogate primary sources just as critically as secondary sources or the opinions of other  scholars and students.

For example, reading a photographic image as text:

• Do we know date, author, location, publication information?

• What is included, such as legible signage, individuals, geographic details, architectural and commercial details?

• Using the quadrants method to read the image, starting in the top left corner and working clockwise, note the details and relation to other visible information.

• Take notes on both your content and your impressions as a historian. The more familiar you are with the background information for the period, location, and subjects, the more context you will be able glean from the image.

• What other sources and other types of material can help contextualize the image and flesh out the historical significance?