The most important thing is to remember about 'the 5 W's':
Who wrote the source or produced it? (Origin)
When? (Origin)
Where? (Origin)
Why? (Purpose)
For whom? Who was the intended audience of the source? (Purpose)
You will need to include all of those in your answer, as well as critically assess each of them. So, for example, if a particular piece of text was written by a Marxist propagandist, you will have to say that it may be biased/is not exactly reliable. The same applies to all of the other types of historians.