When comparing two texts, where should I start?

Comparing two texts is notoriously hard, especially if at first they don't seem to have anything in common. Unfortunately, I have some bad news for you: it's a skill you're going to keep needing for a long time yet. So let's break it down, and keep it simple.

There are two aspects to analysing any text, be it poem, story, fiction, or non-fiction. Technique, and emotion. The first inevitably leads to the latter. By analysing the structure and words used in a text, you can understand where the emotions come from. Short clauses with uninspiring words can inspire cold anger or fear in the reader, where multi-clause sentences with emotive phrases show excitement or sheer fury.

So look at your first text and think, What techniques has the writer used? And what emotion is it making me feel? Look at the sentence lengths, the words used, the punctuation used. Discourse markers in dialogue, whether it's direct speech or not, whether the piece is in first person or uses an omniscient narrator. When you have that noted, then figure out what emotion these techniques cause.

Having looked at your first text, now do it all over again with the second. Then, it's a matter of connect the dots.

Here's a secret: you don't just have to find similarities. Discrepancies are just as important. So they both have short sentences, but it creates difference effects in the reader: WHY? They both make you feel fear, but one uses emotive language and the other doesn't: WHY? Or maybe, even, one has long clauses and happy feelings and the other short clauses and angry feelings - there is even a why to be discussed in this complete lack of similarity! Where's there's a 'why', there's a point. And remember, everything you read is the result of a conscious decision on behalf of the writer, and that is something you in turn can write about.

After that, your format is easy. Look at the techniques in the first text, then the emotions. Then compare the techniques in the second text, and compare the emotions. You now have four paragraphs minimum, and hopefully more than a few 'whys' solved. Try this out with a couple of texts until you get the hang of it, and soon enough you'll be an expert.