Accidentals (sharp #, flat b and natural : ) alter the pitch of the next note and notes written on the same staffline up to the next barline. Accidentals written at the start of a staffline denote the key in which a tune is written, and are valid for all following bars, up to a key-change (if any).
In keys with lots of sharps or flats double sharps and double flats will be used occasionally.
In non-western music and some contemporary music half sharps and half flats are used sometimes
Accidentals may be entered with the accidental-menu
Courtesy accidentals may be used when you feel they are appropriate