It's difficult, but - start early. The sooner you start thinking about the topic of the essay, the more time you will have to collect material. When your brain is tuned into thinking about a question, the information seems to catch your eye.
The same thing happens when you memorize a new word: it starts to seem that it occurs more often in the text. The point is that you become more receptive to a certain type of information, paying more attention to it. If you can't write an essay, you can always contact the free essay writing service. Good luck.