Get comfortable, set a timer for a couple minutes if you want, and just start writing whatever comes to mind about your topic. You could make a list or write in sentences or fragments. Most important, DO NOT EDIT yourself or pause for too long.
To come up with a topic, ask yourself these questions:
What are some things I enjoy?
What have I read about lately or seen on TV?
Is there anything interesting in the news?
What's something I've always wanted to know more about?
What's something I disagree with?
What do I see going on around me daily?
Once you have a topic, try some of these questions to find a more specific direction within your topic:
Is there something controversial about the topic? What are the different perspectives? (hint: a good topic generally does not just have two sides)
Do I agree or disagree with the different perspectives?
What are the main characteristics of my topic?
What is my topic similar to?
What does my topic make me think of?
If my topic is a problem/issue, what is the potential cause of it?
Start with your general topic or argument and branch out from it into every sub-topic you can think of. This mapping method will help you organize your points once you start writing your draft.