To play the note G , pick the open 3rd string (no fingers placed behind the frets).
To play the note A , place the second finger of your left hand just behind the second fret of the 3rd string.
You now have two G notes; the one above and the one at the third fret on the first string. This type of repetition occurs with all notes, since the musical alphabet goes from A to G, and then back to A again. The distance between the two G notes is called octave.