Explain how and when you decided to go to school and apply for a change of status. Since you did not intend to be a student at the time of your visa application at a U.S. Embassy or Consulate abroad, describe very carefully how you came to change your mind. You should not have reached such a decision quickly; rather, the decision should be based on new information you received from reading, speaking to friends from home, or talking to other people.
The immigration officer who reads your application may suspect that you knew you were going to be a student before you entered the U.S. If they suspect that you knew, they may consider your original application for your current visa status fraudulent and deny the application.
Since you changed your mind after arrival in the U.S., you must be very clear about what influenced your decision. In your letter, you should explain your specific academic and professional intentions. You should also address such questions as "What is the highest degree you plan to earn? What is your field of study? Why is it better for you to study in the United States than in your home country?"
You should also demonstrate how being educated in the United States will make a better life for you in your home country. It is critical that you confirm that your intent is to obtain an education and then return to your home country. Keep in mind that nonimmigrant student (F-1) status is a nonimmigrant classification and that your plans must be to remain temporarily in the United States.