I'm confused about the things I need to analyse. Do I need to focus on all three of the things-evidence, reasoning, and style- the author might use, or can I focus on two of these things?
No. You should only focus on the elements that are most prominent in the passage. If the author mainly uses evidence and reasoning, you should analyze that; if the author mainly uses stylistic and persuasive elements, you can focus on that. You'll need to write about multiple different examples to prove your point - for instance, you can't just write about one place where the author uses persuasive language and call it a day - but there's no reason you need to write about all three types of argumentative strategies, and in fact, your essay will likely be weaker if you do this.