I’m trying to identify the first instrument on the first seconds before the singer sings on the song of Green Day, 21 guns? What could be?
The lower notes sound like they are played by a cello.
I think the higher notes are played by an electric guitar with a volume swell - this is so that the sound of each note softly sweeps in and out, rather than it just begin with an instant 'attack'. This volume swell may have been achieved via the guitar's volume knob or a pedal, or using automation within a DAW.