Okay. . .ask yourself this question: "Why do I want to be a nurse?"
Then write the essay based on your personal reasons for becoming a nurse. Some people want to become nurses for altruistic reasons while others are in it for the money and perceived job security. Since everyone's reasons for wanting to become a nurse are different, the admissions people at your school want to know why you want to be a nurse.
Also, they will be checking your writing skills. An essay with too many misspelled words, bizarre phraseology, grammatical errors, and poor construction will be more of a hindrance than a help.