It's hard, but - start early. The earlier you start thinking about an essay topic, the more time you'll have to gather material. When your brain is set on thinking about an issue, it's as if the information itself comes to myhomeworkdone reviews.
The same thing happens when you memorize a new word: it starts to seem like it occurs more often in the text. It's because you become more receptive to a certain type of information, paying more attention to it.
For starters, it's best to make an outline of what you know about the essay topic: you may know even more than you thought you did. This will give you ideas about which direction to go next. To act with purpose, make a plan and formulate a preliminary set of questions. When you start studying the material, you will have new, more specific questions, and you can look for answers to them already.