The easiest place to begin is with your feelings: You have a strong emotional response to something that happened, or something someone said or did. Perhaps it's a political issue, or something to do with civic engagement (my favorite metatopic). Probably, however, it's much more personal, and you can find a topic in anything you've had a strong reaction to. Virtually anything will work, and if you've had personal experience with the subject, you will be writing from a place of authority, a powerful ethical component of effective argumentation.
Curiosity: There's no better fuel, after your emotional response, than a subject you would like to know more about. Usually, this requires an extensive amount of research, and it might even mean you have to go and experience what you're curious about personally. Eventually, your curiosity will lead to experiences that you will have emotional responses to, and a cycle of interest--invention--creation will begin. Curiosity always feels very exciting, and it's the surest way to stay invested in a subject.
Aporia: Aporia means "an irresolvable internal contradiction or logical disjunction in a text." What happens is that you read something and your response is that it makes no sense. Using aporia as a method of approach to find a topic is extremely effective, especially in academic or essay writing. Your goal is to answer the question you've discovered the author has not answered, or has brought up in an incomplete manner. Aporia explains the fundamental problem most readers have with a text they question. For most readers the writing gives them a weird feeling that they're missing something, but the problem might not lie with your reading of the text. The problem might lie with the author's lack of argumentation skills or insufficient or incomplete evidence.