When my son graduated from high school last year, I warned him early and often about the fact that he was going to have to write graduation thank you cards. We ordered them with his graduation announcements and address labels, so we had everything we needed on hand early. And when it came time for him to start writing them, I told him I had just one requirement: that he write more than two sentences in each note.
When it comes to graduation thank you card wording, the first two sentences are the easiest. Thank the person for the gift. Tell them how or where you plan to use it, or how much you will enjoy it. The third sentence requires some thought, but is usually the most interesting and meaningful part of the note. I suggested that he try to make a personal connection with the gift-giver, possibly mentioning an experience or memory they shared, or offer a simple thank you for their friendship over the years.