Bismillāh ir rahman ir rahim
In the name of God, Most Gracious, Most Merciful
An intention is an action of the heart. The fasting person should resolve in their heart that they will fast the following day; this act is better known to Muslims as a Niyyah. The supplication that is advised before any positive action is the Bismilla.
اللَّهُمَّ اِنِّى لَكَ صُمْتُ وَبِكَ امنْتُ وَعَلَيْكَ تَوَكَّلْتُ وَعَلَى رِزْقِكَ اَفْطَرْتُ
Allahumma inni laka sumtu wa bika aamantu wa 'alayka tawakkaltu wa 'ala rizq-ika aftarthu
O God! I fasted for You and I believe in You and I put my trust in You and I break my fast with Your sustenance.
The Prophet (may the peace and blessings of Al'Mighty God be upon him) used to recite this supplication as it was time to break his fast. He used to break his fast with dates (both old or fresh) then he drank some mouthfuls of water.