This is the second topic of the Learning the Musical Dynamics lesson.
The most common and comfortable starting positions for your left hand mirror those for your right:
Your left pinky (also known as LH5) on the C an octave below middle C. The other fingers on your left hand rest on the nearest D, E, F, and G, respectively.
LH 5 moves down to G, the bottom line of the staff, and your other fingers move accordingly.