Choosing Sources
The first question which you should ask is whether your source is a primary or secondary source.
A primary resource is one that is a direct product related to the topic of your research. It can be (1)a lab report giving the results from an experiment, (2) the survey results froma political poll, (3) a transcript of a speech or interview, (4) a film or painting, or (5) an autobiography or report from an eyewitness. There are other types of primary sources, but you get the idea.
A secondary source is one that comments on a primary source.
Usually you want to avoid sources which are tertiary, those which comment on a book or an article which is already secondary. (Read the book yourself. Find the article and read it yourself if you can.)
In other words, if you are writing about Hitler, Mein Kamf would be a primary source, whereas the comments in a recent video about Hitler would be a secondary or even tertiary source.