An essay like this depends very much on whether you have an idea of what you want to be or whether you are still thinking about what you want to be when you are grown up. For example, I always knew that I wanted to be a teacher and it was the right choice for me. My daughter, however, thought that she wanted to be a doctor, went to South America as an exchange student, and became a Spanish medical interpreter. For you, the choices are wide open.
First of all, narrow your topic to either what you want to do or what you think sounds like a good idea for you. I would suggest that you research jobs not often thought of such as a medical illustrator or a CNC mechanic. This is a chance to think about what you enjoy, what you are good at or what you think will fit you.
As for the essay, pick out what you will use and find three reasons why you want to be that person. If you picked medical illustrator, perhaps three reasons might be that you love drawing, you like to help people, and you want to go to college and have an interesting career. These would become your body paragraphs while your introductory paragraph would use these three ideas in your thesis statement. Remember the conclusion paragraph refers back to the introductory paragraph and your thesis statement.
Good luck on your chance to think about your future. Remember that it may or may not include college, but I would include some kind of training if possible.