The Prayer of Fasting Intention
With most things in Islam, like going for the spiritual pilgrimage of Hajj or even praying the five daily prayers, fasting starts with making a du’a, or prayer, for intending to fast, which is called the niyyat. This prayer serves as a notice to God that you intend with your heart and soul to fast during Ramadan for His pleasure. It can be done once before the month begins, or it is recommended to be done every morning at suhoor, the pre-dawn meal before fasting begins.
You basically just state to yourself (as God is your witness) your intention to fast, in whatever language you speak. For example, you can say to yourself, “Oh Allah, I intend to fast today in accordance with your laws and for your benefit. Please accept my fast, forgive my faults, and bring me closer to you.”
Source: “Fasting Rules from Islamic Laws” by Ayatullah Seestani and “Lectures on Fiqh” by Maulana Sadiq Hasan