In The Crucible how does John Proctor move past his sin from the beginning and become a good man?

The first thing that Proctor does is cut off his
relationship with Abby; we see his determination to end it and keep it that way in the
very first act.  When Proctor comes into the house to assess the Betty situation, in a
moment alone, Abby sidles up to him and starts talking to him.  She really wants him
back, but he firmly states, "I will cut off my hand before I'll ever reach for you
again."  You can't get much clearer than that.  He stands his ground, even when she is
flirting with him, and even when she gets angry and hisses insults at his
wife.

The second thing that Proctor does to move away from
his sins and find his own goodness is that he does all that he can to save his wife and
other people's wives from the clutches of the witch hunt.  He goes into town with a
petition, with Corey's statement, and with his own encouragement of Mary Warren.  He
leads all of these charges against the court and against the accusers.  This is a brave
thing to do, especially considering how much power the courts have, and the girls' power
to accuse anyone who displeases them.  Doing such a courageous thing helps him gain
confidence in his goodness.

Another very brave and good
thing that John does is confess his sin, in public, to the entire court and town.  And,
he does so that he might save his wife from being hanged.  In front of the entire
courtroom, he confesses about Abby, "I have known her, sir," an admission of his affair
with her.  To defile his name, renounce his membership of the church, and connect
himself to a lying, conniving girl (Abby), all to save his wife, is an act of extreme
integrity and bravery.

Last of all, and most importantly,
John Proctor does not confess to witchcraft--to a lie--and hangs on the gallows.  This
final testament of honesty and courage finalizes his repentance, and allows everyone to
know that he will do anything--including die--to do what is right.  As he makes that
final decision, he finally reaizes that "I see some shred of goodness in John Proctor." 
With that peace in his heart, he goes to the gallows.

I
hope that those thoughts helped; good luck!