What are the first things I should think about when comparing unseen texts?

PAF

P - Purpose. What is the text trying to achieve? How is it likely to make the reader respond?

A - Audience. Who is the intended reader of the text? You can think about gender, race, age etc. here as well.

F - Format. What does the text look like? How is it organised or structured? Are there any visuals? What are all of these things doing?

You should also think about context where it's relevant, particularly if it is an old document published at a significant historical moment.