how to write speech for when I grow up I want to be doctor

Such questions need to be attempted on your own to test your creative writing skills. However, these points might help you elaborate:
 
- I want to be a doctor when I grow up.

- I have always had interest in treating and curing people; as a three-year old I used to role-play as a doctor with my dolls serving as the patients.

- It is a noble profession in which service comes before self.

- I have started studying very hard, taking special interest in Biology so that I am well prepared.

- My dream is to be able to open a charitable hospital one day for the under-privileged section of society.