Learn how to identify the key signature for the G major scale by memorizing the number of sharps in the scale. The G major has only one sharp, F-sharp. To play F-sharp, press the first key on the upper joint with your left hand.
Play only one octave to begin with and use a fingering chart to help you find the correct fingerings. Start with the middle octave by playing the G above middle C. All of the keys should be open when you are playing a G.
Practice moving from all open keys on the A, to the mostly closed keys on B. The transition between open and closed keys creates a drastic change in sound and explains why the break is so difficult to play.
Add the other octaves, using the fingering chart to ensure that you press the correct keys.