Address the letter to your principal at the school. Sending a letter to her home is a bit too personal and may make your principal feel uncomfortable.
Write your letter's salutation with formal overtones. Keep in mind you are addressing an authority figure and a person who is in a position of leadership and deserves your respect. Write "Dear Sir" or "Dear Ms. Jones" and keep the principal's professionalism intact. It will be appreciated.
Give a quick update of how you are doing. Keep it concise; talk about your classes and how you are achieving the goals that the two of you discussed during your high-school career.
Tell your principal what subjects you are finding a challenge. He may offer advice as to how to navigate the difficulties of certain subjects. All principals have at least a master's degree, so they've been there and done that and can help you out with pertinent advice.
Add a bit of gratitude. Remember that being a principal can be a thankless task: long hours, lots of responsibility both away and at school, dealing with difficult parents and students, and little pay. Thank your principal for all that she's done for you along the way.