You will see a note on the staff that coincides with a natural note on the first three strings in first position. Identify the fret where it is played. FYI: there is a B note on both the second and third string in first position. On the second string it is the open string, but on the third string it is at the fourth fret. For this reason, you may answer 0 or 4 for any B note.
*If the time limit is stressful, try Practice Mode.