Hold the viola with the left hand. The left hand should be wrapped around the next. The index finger should be on the first node of the G string. The remaining 3 fingers should be placed on the remaining nodes moving towards the bridge of the viola. The nodes are the metal frets that appear along the viola.
Press down firmly on the fourth node, using your ring finger. This is the correct place to press down on the G string to play a D.
With your right hand, pull the bow across the string midway between the bridge and the fingerboard. Provided the violin is tuned correctly, this will play a D.