If you committed a mortal sin at some point in your life, and you are sorry, mosey on over to confession next Saturday and confess it to those to whom Jesus entrusted his own authority to forgive sins.
Jesus breathed on the first leaders of the Church and said to them, "Receive the holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained."
Those leaders (the Apostles) then passed this authority down to their successors, who kept on passing down this authority to the leaders of the Catholic Church today!
What would be a mortal sin? There are lists of mortal sins available on the Internet, but here are a few for starters: stealing (if itís more than a few bucks), withholding the wages of an employee, murder (physical or destroying a personís reputation), all the sexual sins (premarital sex, adultery, contraception, masturbation, mutual masturbation, selling pornographic videos, etc.), missing Sunday Mass without serious reason, severe greed, jealousy, gluttony, drunkenness, etc., condemning an innocent person, cursing or hating God, wishing evil or damnation upon another, and receiving Communion unworthily (with an unconfessed mortal sin on our soul).