Catholics believe in God.
Catholics believe God is a Trinity of Father, Son and Holy Spirit.
Catholics believe Jesus was the Son of God, and came to redeem the world from sin. Catholics believe everyone is born with Original Sin, which is passed down through the human race from Adam and Eve, and their disobedience to God in the Garden of Eden.
Catholics believe that by loving God, and following the teachings of the Church, and by receiving the sacraments and following the Golden Rule, they will be good people, and merit Heaven when they die, perhaps after a period of purification in purgatory. Those Catholics who feel uncomfortable about their sins confess them to a priest in the sacrament of reconciliation.