Sin separates us from God. It is as if we are turning our backs on God. We need therefore to turn back to him and ask forgiveness.
Sin is anything that is selfish or unloving. It including big crimes such as murder, but also includes more common things which we all do - losing our temper, ignoring the needs of others, lying, being dishonest and so on. Jesus told us to love God and to love our neighbour as ourselves - every time we fail to obey this simple law we fall into sin.
But God does not want us to be separated from him. So he offers us forgiveness in Christ. There can be no greater example of sin than that the Son of God was crucified - but Jesus died in love, praying that God would forgive even those who killed him. His love is stronger than sin and death, and therefore his love is stronger than any sin we could commit.
The principal way of forgiveness is to come to Confession. Here a person tells God their sins and asks forgiveness. God, working through his priest, pronounces absolution (forgiveness.) Of course God can only forgive us if we are sincere, but if we truly want his forgiveness then we can receive it.