The 7th chord (correctly called "dominant 7th,) is formed by adding a flattened 7th note to the major chord:
The C7 chord is constructed from the C major scale as such:
The A7 chord is constructed from the A major scale:
Remember to check the sharps or flats involved in each scale when working out the notes of a chord. Construct the following 7th chords: D7 G7 E7 Bb7