Spend some time brainstorming your options. Think about which field you are most drawn to and why. Think about the various occupations within that field and which one is most suited to your particular personality. If you are interested in politics, for example, you could be a professor of political science, a lobbyist, a politician, a journalist or a lawyer. Imagine yourself in each of these various roles and choose one you would like to explore further.
Be specific. A decent essay may focus on your desire to become a doctor. A better essay will specify that you would like to be a neonatologist who works in the newborn intensive care unit. Being specific about your career goals will help your audience better understand what you want to say.
Discuss personal reasons for your choices. Giving this personal background information can make your essay more compelling. For example, perhaps you would like to be a research scientist so you can help find a cure for a disease that affected someone close to you.
Research your chosen occupation. The more knowledge you display about a given field, the more powerful your essay will be. Cite some statistics about the field and concrete examples of what notable professionals have accomplished. Read some recent news items to find out about about new developments in the field. Write about those innovations and what the future may hold. Add thoughtful insight about what you need to do to achieve your goal, especially the training and education that goes into securing a position in your profession.
Speculate as to how the field may be different by the time you grow up. For example, if you would like to be an architect, you may want to weigh in on the trend for firms to use computers to design buildings instead of humans. You can also weigh in on current controversies within the field and take a position. For example, if you want to be a psychologist or psychiatrist, you may consider and discuss the concern that patients are given too much medicine.