When sight-reading using the solfège syllables (Do, Re, Mi, etc.), the starting note:
• Is typically Do, unless there is an accidental (sharp/flat) next to the starting note.
• If there is a sharp next to the starting note, you would use the solfège syllable for the next higher note in the scale, e.g. a C sharp would become De.
• If there is a flat next to the starting note, you would use the solfège syllable for the next lower note in the scale, e.g. a C flat would become Ces.
So sharps and flats will adjust the solfège syllable for the starting note. For example:
• C - Do
• C sharp - De
• C flat - Ces
• D - Re
• E flat - E
• E - Mi
• F sharp - Fi
• G - Sol
• A - La
• B flat - Si
• B - Ti
Does that help explain how to determine the solfège syllable for sight-reading music with sharps and flats? Let me know if you have any other questions!