My best suggestion is that you choose something that you feel passionate about. If you just pick any old problem just to get the assignment done, your essay might lack a clear voice. Pick something that bothers you, something you’ve thought about a lot. Stay away from the huge global problems that you have no idea how to solve. Stick with the more personal things that someone your age might encounter on a regular basis. For example, you go to school– what are some serious problems there? Or there are certainly problems with your generation that you would like to see change. It could be something as ordinary as improving school cafeteria food to something as serious as drug and alcohol use or anorexia.