In Lesson 11 you were introduced to the major scale sequence of tone - tone - semitone - tone - tone - tone - semitone, which results in the familiar sound of DO RE MI FA SO LA TI DO. If you commence this sequence on a G note, you will be playing the G Major Scale.
You will notice that in order to keep the interval sequence correct, the G major scale contains an F# note. Here is two octaves of the G major scale.