“…If you bring your gift to the altar, and there remember that your brother has something against you, leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift.” John 5:24  

So, we see, first we apologize to whomever we have wronged, and do whatever is necessary to make amends, to the best of our ability. Then we may be assured that “If we confess our sins, He is faithful and just to forgive our sins and to cleanse us from all unrighteousness.” 1 John 1:9.  Once we've done our best to “clean up our messes”, God's mercy, through the merit of Christ's blood makes up the rest.