When a chord is played one note at a time, as in bars 1 and 3, it is called an arpeggio.
This is a sharp sign.
When a sharp sign is placed before a note on the staff, it means that you play the key immediately to its right. This key may be either black or white.
F#
The note F#, is the black key immediately to the right of the F note as shown in the diagram.
D7
The D7 chord contains the F#note given above. To play the D7 chord use the first, second and fifth fingers of your left hand, as shown in the D7 diagram.
When a sharp sign is written before an F note, it applies to all F notes which come after the sign in that bar. Thus, bars 3 and 7 both contain two F# notes.