To play the note Low D, use the thumb, first, second and third fingers of your left hand, also the first, second and third fingers of your right hand. It is very important that you cover each hole completely with your fingers, otherwise the note will not sound properly.
Sometimes, as in Twinkle, Twinkle Little Star, there are two sharp signs after the treble clef. You do not need to learn any new notes to play this song.