Psalm 100 is a common Thanksgiving prayer that includes the saying, "Serve the Lord with gladness: come before his presence with singing. Know ye that the Lord he is God. Enter into his gates with thanksgiving and into his courts with praise."
A Thanksgiving prayer could be personal, adding the blessings that you and your family received throughout the year. Joanna Fuchs also has numerous examples of Thanksgiving prayers including, "Dear Lord, we come to You today with a humble attitude for all you give, our hearts are full of love and gratitude. Thank you for this special time to offer our Thanksgiving. Thank you for this food, and for the blessed lives we're living. In Jesus' name we pray; Amen."