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?
One published guitar transcription sheet music directs these opening 4 measures of half note dyads (intervals of octave + maj 3) are to be played on an electric guitar with volume swells from p > mf for each note, with clean tone and delay, with fingers (no pick).
There's not much to go on, but to me it sounds quite like synthesized music (i.e. using a synthesizer with electronically generated sounds). The second and forth chords sound quite like lower stringed instruments, but they could easily be electronically generated as well.
There are two instruments playing a third apart. The lower notes sound like a cello. I can't place the higher one with the ringing sort of sound. It might be non-Western, ancient, or digitally altered.
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.