Write a proper greeting or salutation. Avoid casual starts or first names unless you are already on a first-name basis with your tutor. The beginning of your letter should start with something such as, "Dear Mr. Smith."
Draft a specific body. Instead of a general thanks, be precise about what you are thanking the tutor for. This may include help passing a test in a particular subject, working with you to raise an overall grade, learning new material or something similar. Note specific details about projects, classes or tests. For example, if the tutor helped you write a report on George Washington, say that you are grateful for the assistance with this assignment and all of the steps leading up to it such as library research or even guidance on spelling and grammar.
Add information or statements on what you appreciate about your tutor. Think of attributes such as patience, helpfulness and willingness to go the extra mile.
Conclude the letter with a proper ending such as "Sincerely," "Thank you" or "Respectfully" followed by your name.