To form a seventh chord, you take a Major or Minor Chord and add another note on top. However, there are several different kinds of the Seventh Chord which you need to be aware of.
Example: This is the scale of G again:
The seventh note of the scale is an F# and adding this to G Major would give a G Major Seventh chord.
If you flatten the seventh note to F you get an 'ordinary' seventh (called a dominant seventh - G7).
If you flatten the 3rd (to make a Minor Chord) and flatten the 7th, you'd have a G Minor Seventh Chord.
Take a look at the table below:
|G major seventh||G||B||D||F#|
|G dominant seventh||G||B||D||F|
|G minor seventh||G||Bb||D||F|