The forgiveness of sins committed after Baptism is conferred by a particular sacrament called the sacrament of conversion, confession, penance, or reconciliation.
"On the evening of that day, the first day of the week," Jesus showed himself to his apostles. "He breathed on them, and said to them: 'Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained"' ( Jn 20:19, 22-23).
photo stained glass in St. Dominic Church (Columbus, Ohio) by Nheyb. Wikimedia, 2014. Free to post for non-commercial purposes.
Information from The Catechism of the Catholic Church on Reconciliation
Contact your parish church for more information on preparation and celebrating the sacrament