for the compulsory fast of ramadan to be accepted, the intention to fast must be made before fajr each day. the prophet (peace be upon him) was reported by hafsah to have said, "whoever does not intend to fast before fajr will have no fast."
as with salah, there is no verbal formula one says to mark one's intention. to wake up for the pre-dawn meal is an expression of the intention to fast. even of one didn't wake up for suhur, to simply make up one's mind before sleeping that one will be fasting the next day is sufficient.
suhur (pre-dawn meal)
the fast begins with a light meal known as suhur, which is taken just before the break of dawn. the prophet (peace be upon him) encouraged his followers to take this meal, even if it is only a drink of water. anas ibn malik said that prophet muhammad (peace be upon him) said, "take suhur for surely there is barakah (blessing) in it."
when the adhan of fajr is sounded, this signals the beginning of the fast and one should not begin to eat anything at this point. if one has in his hand food or drink when the adhan is called, he should finish eating what he wants from it before putting it down. abu hurayrah reported that allah's messenger (peace be upon him) said, "if one of you hears the adhan and a vessel remains in his hand, he should not put it down until he has had what he desires from it."
iftar (breaking fast)
it is highly recommended that the fast be broken as soon as maghrib (setting of the sun) occurs. the prophet (peace be upon him) said, "people will remain happy and prosperous as long as they hasten to break the fast."
it is also recommended that the fast be broken with dates and water if available. anas ibn malik said, "the prophet (peace be upon him) used to break his fast with a few dates just before the beginning of salatul-maghrib. if fresh dates were not available he would break his fast by drinking a little water."
du'a for breaking the fast
(thahabath-tham'u wab-tallatil-‘urooqu wa thabatal-ajru inshaa' allah.)
"the thirst has gone, the arteries are moist, and the reward set, allah willing."