Admissions Letter

Hello
I am looking to apply for University, but I have not had the best grades in high school, or college. I am currently upgrading my 12 U courses. I have been informed that I will need to include an admissions letter to my application, but I have no clue as to what I should include in my letter. I understand that I am telling my own personal story, and why I have chosen this school. I am not sure how personal is too personal, or what would make me stand out.