How would you start an essay titled "What do you want to be in in the future?"I want to be a doctor when I grow up... my teacher told me I just...

You should first give a brief introduction about yourself.
You could mention what kind of environment you grew up in, who influence you most in
your life or etc. Adding an anecdote on why you became to have a dream as doctor would
help. Then at the end, state what a doctor does that appeals to you the most. If you
link that with your personal life, then voila! You have your one paragraph. Remember,
it's more important to be honest than to be impressive; writing from your true feelings
will be more easier than creating a dramatic, unrealistic story.