Imsak means to stop eating and drinking when the dawn comes. Stopping eating and drinking ten minutes before Fajr is not correct.

When a person becomes certain that the true dawn has come, he has to stop eating and drinking, and if there is food in his mouth he has to spit it out; if he does not do that, then he has invalidated his fast.

The true time of Fajr refers to the appearance of the second dawn according to consensus. That is indicated by the reports of the times of prayer. The second dawn is the white line that appears along the horizon; it is called the true dawn (Al-Fajr As-Sadiq).

1- You should say this du`a after sighting the new moon of Ramadan and other Hijri months: ‘Allahumma ahillahu ‘alayna bi’l-yumni wa’l-iman wa’l-salamah wa’l-islam. Rabbiy wa rabbuka Allah.’ 2- There is no du`a that you should say when starting to fast each day. Rather you should simply have the intention that you are going to fast tomorrow.