What is the most important thing that you need to consider in writing an essay?

It depends. (check explanation)

Explanation:

To be honest, it depends on the essay you're writing. If it's a persuasive essay, then you need to focus on making your points solid and undebatable. If it's a research essay, you need to focus on citing a lot of sources. However, one thing needs to be in common in all your essays that teachers look for. Evidence. Evidence is key to developing your essay and making strong points. Most teachers look for evidence in your paragraphs and they will make your writing much more professional and give you a base to build your ideas off of. I hope this helped, please let me know if you have any further questions. :))