The ball of your left thumb should be on the thumb rest when you play. Don’t press too hard with your thumb, as you will need it later to play the octave key.
A white number on a red key tells you to press down the key with the finger indicated. Press down the three red keys indicated on the diagram to play the note G.
The right hand does not press down any keys when playing a G note. Always keep the tips of your fingers hovering near the keys, ready to play the next note.