The following comes from Blessed John Paul II's Letter to Youth:
We could speak here of the "life" vocation, which in a way is identical with that plan of life which each of you draws up in the period of your youth... This "plan" is a "vocation" inasmuch as in it there make themselves felt the various factors which call. These factors usually make up a particular order of values (also called a "hierarchy of values"), from which emerges an ideal to be realized, an ideal which is attractive to a young heart. In this process the "vocation" becomes a "plan," and the plan begins to be also a vocation.
...During youth a person puts the question, "What must I do?" not only to himself and to other people from whom he can expect an answer, especially his parents and teachers, but he puts it also to God, as his Creator and Father. He puts it in the context of this particular interior sphere in which he has learned to be in a close relationship with God, above all in prayer. He therefore asks God: "What must I do?", what is your plan for my life? Your creative, fatherly plan? What is your will? I wish to do it.
In this context the "plan" takes on the meaning of a "life vocation," as something which is entrusted by God to an individual as a task. Young people, entering into themselves and at the same time entering into conversation with Christ in prayer, desire as it were to read the eternal thought which God the Creator and Father has in their regard. They then become convinced that the task assigned to them by God is left completely to their own freedom, and at the same time is determined by various circumstances of an interior and exterior nature. Examining these circumstances, the young person, boy or girl, constructs his or her plan of life and at the same time recognizes this plan as the vocation to which God is calling him or her.