Which topic should write my essay on?

This is going to depend on the specific college application. The Common App and Coalition have their own prompts (with some prompts that overlap) and those don't change, so look through those and choose the question that speaks out to you and that you could write a solid essay about. The UC system has their own application with 8 different questions that you have to choose 4 to answer. Most colleges also have additional short essay questions you have to answer.

Whatever application for the specific colleges you're applying to will tell you the prompt. After reading the prompt, think of possible topics you could write about that would a) answer the question/prompt, and b) is something that is true to you. For example, if the prompt is asking you what made you want to study a specific major, you could write about how you were curious about the subject when you were younger and the interest only grew.

Just make sure the topic you end up writing about is something you are truly passionate in because college admissions officers can tell when you're writing about something just for the sake of it.