Mother Teresa first had a vocation to be a nun. So she joined a religious order and went to India to teach.
Then she had another vocation, a second calling from God, as she tells.
She felt this calling was to emulate Jesus and his compassion for the poor. Saints and sages teach us that Jesus literally wore himself out in compassion for the poor.
Mother Teresa decided Jesus was her role model, and she would spend the rest of her life, seeking Jesus (whom she believed was in the hearts of everyone she met) ... but especially, in the poorest of the poor.