This is a sharp sign.
A sharp sign written before a note on the staff means that you play the note that is one fret higher than the written note. The following diagrams demonstrate two F sharp (F#) notes. When a sharp sign is written on the staff it is always written before the note.
The note on the second fret of the first string is an F# note. Use the second finger of your left hand to fret this note. This F# note is notated on the following staff.
The note on the fourth fret of the fourth string is also an F# note. Use the fourth finger of your left hand to fret this note. This F# note is notated on the staff as shown in the diagram.