The index, middle, ring, and little fingers are the playing fingers of the left hand.

They are labeled in standard music notation as follows:

index - 1
middle - 2
ring - 3
little - 4

The left hand thumb is not a playing finger.

To bring the left hand to the fretboard in the most efficient manner, assume your playing position and relax your left arm at your side.  Now, rotate your arm so that the left hand is facing forward and bend your elbow until your fingers reach the fretboard.

Your thumb should be behind the neck, perpendicular to the floor.  To balance the hand, the thumb should be placed between fingers 2 and 3 (on the opposite side of the neck).  In first position, this would put the thumb
behind the second fret proper.  The tip of the thumb should not be any higher than the top side of the neck.