So, you've been given the freedom to pick *whatever topic* you'd like to talk about! Excellent! The first bit of advice is.... don't panic.
It's a little intimidating to have the whole wide world to choose from, but here are a few guidelines to help you get started...
1. Define Your Purpose
The first thing you must do is think about the purpose of the essay you must write. Is your purpose to persuade people to believe as you do, to explain to people how to complete a particular task, to educate people about some person, place, thing or idea, or something else entirely? Whatever topic you choose must fit that purpose.
2. Brainstorm Subjects of Interest
Once you have determined the purpose of your essay, write down some subjects that interest you. No matter what the purpose of your essay is, an endless number of topics will be suitable.
If you have trouble thinking of subjects, start by looking around you. Is there anything in your surroundings that interests you? Think about your life. What occupies most of your time? That might make for a good topic. Don't evaluate the subjects yet; just write down anything that springs to mind.
3. Evaluate Each Potential Topic
If you can think of at least a few topics that would be appropriate, you must simply consider each one individually. Think about how you feel about that topic. If you must educate, be sure it is a subject about which you are particularly well-informed. If you must persuade, be sure it is a subject about which you are at least moderately passionate. Of course, the most important factor in choosing a topic is the number of ideas you have about that topic.
Even if none of the subjects you thought of seem particularly appealing, try just choosing one to work with. It may turn out to be a better topic than you at first thought.
Once you have determined that your topic will be suitable, you can move on to the next step: narrowing your topic.
(hat tip to Kathy Livingston)