You could start as" Once in my life, I had a dream of being (becoming)..(write down whatever the person or career you want to be or want to pursue)
Then you could use your own imagination to think about what the person's personality, and in what case do you admire about that kind of person?
Or when you want to say you had a dream of becoming your own boss, then you can write down some benifits of being your own boss, which is the dream about to tell people the brillient side of the dream?
You can have a dream in the past or in the future, which totally depends on you?
I do hope this will help you a bit.
Good luck with your assignment.