letter to professor to get exception from exam

hey, i really need someones help. i am medical student, and i had some of the courses before i came to this university that i am studying now. so i asked the office to give me exception from the exam on the course that i had, and i got to know that i have to write an english letter to the professor saying that i want exception from that particular course. and the professor will decied if he/she like my letter. it means the ltter should be excellent , i mean very smart way to dcripe myself and my ablity, knowlegde. because if the letter is good then the professor can give me exception. please someone help how to start and how to impressed my professor with this letter.
thanks in advance.

regards tina