Ths dua should be read during the first day of Ramadan. It is not mandatory to read this dua everyday because on the first day of Ramadan you are intending to fast for the whole blessed month.
Dua For Sehri and Dua For Fasting
وَبِصَوْمِ غَدٍ نَّوَيْتُ مِنْ شَهْرِ رَمَضَانَ
Wa Bisawmin Ghadinn Nawwaiytu Min Shahri Ramadan
Meaning: “I intend to keep the fast for tomorrow in the month of Ramadan.”
Dua to Break Your Fast and Dua For Iftaar
ذَهَبَ الظَّمَأُ وَابْتَلَّتِ الْعُرُوقُ وَثَبَتَ الأَجْرُ إِنْ شَاءَ اللّٰهُ
Meaning: Thirst is gone, the veins are moistened and the reward is certain if Allaah wills.
The above Dua is mentioned in Hadith No 2357
Book of Fasting (Kitab Al-Siyam) By Sunan Abu Dawood