Given your number of topic posts, you should probably prepare twice as many as everyone else.
However, in one of my rare helpful moments, I'll try to give you an answer. Topics change from board to board, and if memory serves the essay is to be about 2 sides of A4 long.
The topic I answered on my main board was about the historian David Irving (he who denied the Holocaust), so it's obviously current-affairs based. As a result, you can't really prepare any essay, only practice writing them, so that'd be what I recommend, along with reading a quality broadsheet newspaper, and something like The Week.
The whole point of the essay, as far as I know, is not to test your current affairs knowledge, but rather to see if you can write in a coherent fashion, and get information across effectively, so practice doing that.
I guess you're still in school, so a potential idea would be to try and write an explanation of how something works, say, how a bike is propelled, then ask a teacher for advice on how to make it clearer and the passage information more effective. It works for some, might work for you.