Fasting means that a person must , in obedience to the commands of Allah , from the time of Adhan for Fajr prayers up to Maghrib, avoid nine thing whicw will be mentioned later.
Niyyat for Fasting:
It is not necessary for a person to pass the niyyat for fasting through his mind or to say that he would be fasting on the following day. In fact, it is sufficient for him to decide that in obedience to the command of Allah he will not perform from the time of Adhan for Fajr prayers up to Maghrib, any act which may invalidate the fast. And in order to ensure that he has been fasting throughout this time he should begin abstaining earlier than the Adhan for Fajr prayers, and continue to refrain for some time after sunset from act which invalidate a fast.