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.