The first thing you should think of when writing a "Thank you" note is to decide on an appropriate "thank you" card to use. Depending on the relationship between you and whoever you are writing this to, you might decide to go for the physical "thank you" card or use the ones on the internet. Start your "thank you" by greeting the receiver using the word "Dear" together with their name.
After this, go to the body of the note by writing a message of gratitude for whatever they have done for you. You should make sure your words are easy to read since you wouldn't want a situation whereby the reader won't understand the purpose of the note.
Do this by checking for possible grammar, spelling, and punctuation mistakes. You should also include in the letter what you are looking forward to from them. Finally, you can end the note using words like sincerely, with love, truth, etc.