The graders of the essay want to see how coherently you can write and formulate an argument. You'll be graded primarily on the structure and organization of your essay, your writing mechanics, the general flow and style of your writing (don't make it flowery and over-embellished, just make sure it reads nicely), and the like. Supposedly factual inaccuracy is not penalized, so while you shouldn't rewrite history, you can have some inconsistencies, as it's not testing your factual knowledge.
It's difficult to really prepare for the essay, but if you're nervous about that portion, I recommend you check out a good SAT book, especially CollegeBoard's Official Guide to the SAT, for sample essay prompts. They are very open-ended, usually the subject isn't bad even if the question or quote is worded oddly, and your examples can be from personal experience, literature, current events, history, etc.