As an amateur historian I'm mostly self taught and concentrate on 1930s-1970s I've learnt a lot but one thing I missed out on from taking real classes is source content. What is the difference between primary and secondary sources? And why is one or the other good to use in a historical context? I'd like to reply here more often but I'm always afraid of not citing properly. I hope this is okay to post in this subreddit because it's in a historical context related directly to responding here.
Simply put, a primary source is an item/letter/document/diary/photograph/video/etc. that was written or created during that specific time you're looking at, they give a firsthand/inside account (usually from an individuals point of view) of the event. A secondary source is a source written after the fact that analyzes and examines the event as a whole, usually constructed from a number of primary and secondary sources. A primary source is good to look at if you want to understand how an event occurred on the ground floor, a single soldiers viewpoint of the battle, the secondary might cover the whole battle or even the war.
Edit: You might also want to look at these questions asked previously:
What are the key questions that should be asked when dealing with primary sources?