There is a problem for many people, it is cannot pluck the right string.
Actually we must put the right finger on the right string anytime, no only when want to pluck it just find which is the right string.
As i say before, Thumb is use to play 4th, 5th, 6th string.
Then finger 123 is plucking string 321.
So just put ur finger like the below picture.
Front view:
Side view: