When Peter realized that Jesus is God in a human body – “the Christ, the Son of the living God” – Jesus told him He would give him the keys of the kingdom of heaven and authority.

"And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven." - Matthew 16:19

Jesus spoke to Nicodemus of the kingdom of God. (“The kingdom of heaven” is interchanged with “the kingdom of God” i.e. Mark 10:14, Matt. 19:14)

"Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God." - John 3:5

While Jesus made it clear we must be born of water and of the Spirit to enter the kingdom of God, he did not give specifics. Jesus gave Peter the keys as to how exactly we are born again. Peter revealed this on the Day of Pentecost after he received the Holy Ghost.

"And when the day of Pentecost was fully come, they were all with one accord in one place. ... And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance." - Acts 2:1, 4

When the people asked, “What meaneth this?” Peter preached the gospel, letting them know they had crucified their Messiah, the One they had been waiting for.

Then the people asked, “Men and brethren, what shall we do?”

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." - Acts 2:38

Peter made it perfectly clear what Jesus meant when Jesus said a man must be born of water and of the Spirit. The keys to the kingdom of heaven are to repent, to be baptized by immersion in water in the name of Jesus Christ, and to receive the gift of the Holy Ghost.

We now understand what we must do.