Practise your G major and E minor scales ascending and descending.

Improvise a melody using the G major scale over this track.
G / D / C / G D x 2
Em / D / Em / C D
G / D / C / G D x 2

Then play chords in that key with melodies without the track.

Improvise a melody using the E minor scale over this track.
Em C  G / Em C  G D x 2
Am D G Am / D /
Em C  G / Em C  G D

Then play chords in that key with melodies without the track