The amplified bible reads in Colossians 3:13, "as the Lord has [freely] forgiven you, so must also [forgive]."
Forgiveness is a free gift from God. This is what Jesus was sent here to do, die on the cross so our sins can be forgiven. Because our sins are forgiven, we can live in heaven with Him forever.
Even though God freely forgives sins, we must do two things. One is we must forgive others. Jesus tells a parable of the unforgiving servant. That parable is in Matthew 18:23-35, if you want to read it. At the end of it Jesus said, "So also My heavenly Father will deal with every one of you if you do not freely forgive your brother from your heart his offenses", (Matthew 18:35 amplified version).
In a nut shell, the parable is about a man who owed a lot of money to the king. When he couldn't pay, the he begged for forgiveness and the king forgave the debt. Later that day, the man had another man who owed him a few dollars (in today's terms) and when this new man begged forgiveness, the other man (the one who the king forgave) didn't forgive him. When the king found out what happened, he imprisoned the man who owed him money until he could repay his debt.
We are to freely forgive others as Jesus has forgiven us. The other thing we need to do is to ask for the forgiveness.
If someone does something wrong to you, forgive them. Freely forgive them, from your heart. In the NIV version, Ephesians 4:26 reads, "do not let the sun go down while you are still angry." Many times that may be hard, but we need to do it.
Put yourself in this situation. If you did something wrong to someone, wouldn't you want them to forgive you?