First, what is the purpose of this writing? It's good for us to know that to provide you with more relevant feed backs. I guess it is just a class room type of essay about your ambition. Is that so?
I wanted to become a doctor when i grow up. My desire to[b] became a doctor[/b] (sounds repetitive) is when my grandfather, who was suffered from cancer died.
I always dreamed of becoming a doctor one day. This desire of mine was nurtured by watching how my grandfather suffered from a cancer.My parents always brought me to visit him every weekend. I was young by that time, I was only 9 years old, I didn't even know what is cancer! So I asked a lot of questions as to why my grandfather need to stay at the hospital for such a long time. At that time, my mother answered me that he was suffered from some sort of sickness which could harm his health.
I feel you need to organize your ideas better. They seem to be a bit sporadically presented. First, note down the main ideas roughly and then arrange them in a more logical sequence. Then start writing sentences.