Once you know the question you need to answer, spend some time doing some general reading about your topic, called 'background research'. The purpose of background research is to improve your background knowledge. This helps you to familiarise yourself with the events, people and places you need to write about.
Things you need to establish during background research:
Important concepts or foreign words
Good background research helps you write better sub-questions and improves your vocabulary, which will improve the overall sophistication of your essay writing.
General history websites or Wikipedia help give you some background to a topic, but do not use them as references or sources in essays.
Below are some good sites to help with your background research:
Ancient History Encyclopedia is a non-profit educational website with a global vision: to provide the best ancient history information on the internet for free.
A website which aims to provide historical information that is easy for both children and adults to navigate, is written in accessible language, is illustrated wherever possible and makes learning fun.
History Skills Online has a ready-to-use Research Journal that follows these 9 steps and provides links back to the website to help you at each stage of your research. You can grab it here.