Primary sources are first-hand accounts of an event or period of time created by participants or observers. There are many kinds of primary sources including texts (letters, diaries, government reports, newspaper accounts, novels, autobiographies), images (photographs, paintings, advertisements, posters), artifacts (buildings, clothing, sculpture, coins) and audio/visual (songs, oral history interviews, films).
These are easy to find and contain reliable, quality resources. When searching full text, you must use the words of the TIME. For example, you cannot search WWI if looking for newspapers from that time - it was not called WWI until WWII happened! You'll have to try things like "Great War" or even just search "war" and use the time period limiters on the side.
Government documents provide an excellent look into the inner workings of political situations, from the perspective of governing bodies. They reveal arguments or rationale behind decisions, messaging intended to sway the public a particular way, and more. Further, they might collect information about the population of a time (census records, tax records) and reveal the bureaucratic inner-workings of societal functions.
Research institutes, universities, museums, and archival repositories hold archival collections that shed light on particular people and events. More and more of these collections are digitized for public access.
National or country archives contain sources that document the history of that countries; these resources are include a multitude of formats: government documents, images, personal papers, and much more.
In addition to general government archives, sometimes specific agencies have their own archival collections. It might be helpful to target these collections to search for primary sources. When reading your secondary sources, pay attention to the agencies involved.