I find it helpful to start with soaking up as much knowledge from secondary reading as I can. Your teacher might have provided you with a reading list or text, which is always a good place to start. Most books will have a 'further reading' section or bibliography, which is the next stop if you're looking for more detailed information pertaining to you specific essay question. However, it's important you don't overwhelm yourself with more information than you need; you're probably working towards a deadline, and reading tonnes is inefficient. Choose your reading selectively. Don't know how? I can help you out here! The next step is to digest the information you have read in the best way that suits your learning style. Sometimes I make notes on paper, or on my computer, and other times I find it helpful to talk it through with someone or draw diagrams. The important thing is that your making connections across what you've read. You can make a rough essay plan if that helps, based on the main points that stand out from your reading. From there, you can start your essay.