Good advice from Awk.
I think at this level they're just looking for evidence that you can communicate in a reasonably coherent manner. Definitely avoid any grammar/spelling errors (obviously). Also don't use abbreviations that are often seen in email (honestly, I have seen essays with things like 'b4' for 'before' and 'cuz' for 'because'). Your posts don't look like this, so I think it won't be a problem for you.
Talk about why you want to go to the school and what your educational/professional goals are long-term. You seem serious & focused on what you want to do, and this will come across positively in your essay. If there is something that you really like about a particular school (for example, you're interested in majoring in history and they have a well-respected history program), it's also a good idea to mention it in the essay.
Don't stress too much about this; it shouldn't be a really big deal at this level. Good luck!