Place the first finger of your left hand just behind the third fret of the G string. This note is a Bb note. Play this new note with your right hand index finger. Listen to the sound. It should sound full and round. If it buzzes or rattles you have not placed your left hand finger close enough to the fret or you are not pressing the string down hard enough.
For greater support the left hand thumb should be placed behind the neck of the bass guitar, approximately opposite your index and middle fingers as shown in the previous photo.