Cain - the first B.T.

 Kayin (Cain) was the first person to do teshuvah and he is as an example for all future Ba’alei Teshuvah. Kayin’s teshuvah consisted of three phases:

a) Confession. Kayin declared to God, “My sin is too great to
bear”. The Torah then states that, “Kayin left God’s presence” (v. 16), on which the Midrash comments that Kayin was rejoicing that his confession had been accepted by God.

b) Exile. Rambam writes that one of the routes of teshuvah is for
a person to send himself into exile, for this generates a spirit of
humility within a person (Hilchos Teshuvah, 2:4). Thus we find that Kayin, “dwelt in the land of the wanderers”.

c) Rebound into Positive Action. There is a tremendous temptation for the Ba’al Teshuvah to remain low-spirited for the rest of his days. The mere thought of his past deeds, which cannot be erased from his mind, is sufficient to give him feelings of inferiority. Obviously, in the midst of such a mood he will find it difficult to be active within the world, being plagued by the eternal question, “Who am I to carry
out a holy activity like this."

Consequently, the challenge of the Ba’al Teshuvah is that when his teshuvah is complete, he must propel himself “outwards” into the world. He must free himself from his feelings of inferiority, and start
to contribute constructively to the world in the most expansive manner possible.

Thus, we find that after doing teshuvah, Kayin propelled himself back into the world:

 a.) He fathered a son.

 b) He built a city—an ambitious project aimed at repairing the world that he had damaged.
And, furthermore,:

 c)  He named the city after his son’s name "Chanoch” I.e. Not only did he free himself from feelings of inferiority, he actually went to the opposite extreme and publicized
his achievements boldly to the entire world.

(Based on Likutei Sichos vol. 35, pp. 7-9