Eh, I guess I could help with spelling/grammar and perhaps word choice, but that's about it. It's about all anyone here will be useful for anyway, unless you're writing on a topic where one of us happens to be an expert.
But really, there's no way of ensuring the quality of something that comes off of one of those websites. You have no idea if you're getting what your paying for or if it'll just be another crappy essay. I've looked at those websites from time to time just trying to think up topics (and nothing else), and from what I've seen, the people there can't write for shit. And who's to say they won't completely misunderstand the prompt? And if the essay somehow did turn out to be well-written (unlikely), like Firestorm said, your teacher is probably smart enough to recognize that it's not your work because it'll be a completely different writing style. It's just totally not worth it on so many levels.
Just write the rough draft, see what you can come up with, and show it to as many people as possible who you think could help you. This includes your parents, your smart friends, and your teacher. In fact, I'd suggest emailing your teacher right away to set up an appointment for extra help. After all, that's what teachers are there for-- to help you learn!! And they're usually happy to see students take initiative to ask them for help when needed.
