How to be born again

In John 3, when Jesus held Nicodemus accountable for knowing that he had to be born again, he probably had Ezekiel 36 in mind by which God promises to give us a new heart and a new spirit so that we can live according to God's statutes. In order for God to do this we first must give him the legal authority by surrendering our heart to him in repentance. Simply believing that Jesus is the son of God is not enough to be born again, we also have to turn all our heart to him in repentance. Acts 2:38 says we have to both "repent and believe". 

Repentance also has to be ongoing and an everyday practice:

Acts 26:20
but declared first to those in Damascus, then in Jerusalem and throughout all the region of Judea, and also to the Gentiles, that they should repent and turn to God, performing deeds in keeping with their repentance.