Open your thank-you note with "Dear Mr. or Mrs." Then, on the first line, start with "Thank you for..." and be very specific about what you're thanking them for. On the second line, tell them how you're going to use whatever that person did for you. For instance, you might say, "Thank you for taking the time to work on my algebra with me after class. Because of your help, I'll do much better on the SATs." End with a warm greeting, like "Wishing you well," "Take care," or "With warm regards," then sign your name.