Brainstorm what you already know. Try to think about topics that you have some knowledge on. It will be easier to write on the topic if you can instruct from experience. Chances are you have already had a go at explaining the concept to someone.
Pick something you want to know more about. The idea is that you will want to write on the topic and have good questions of your own that you can find the answers to and present in your essay.
Talk to your friends and family. If you get stuck, ask your friends or family to tell you something they might want to know more about. You may find a common idea that you did not come up with on your own.
Listen up and look around. Some great ideas can be found on the news, in papers and magazines. Plus, these sources usually supply a lot of the information you may need to support your thesis and main ideas.
Ask your teacher. Your teacher will have a good idea of what topics may be on the actual OGT and can point you toward good resources or topics.