A few scriptures to start with...
John 3:3-7 (The Message) 3 Jesus said, "You're absolutely right. Take it from me: Unless a person is born from above, it's not possible to see what I'm pointing to--to God's kingdom."
4 "How can anyone," said Nicodemus, "be born who has already been born and grown up? You can't re-enter your mother's womb and be born again. What are you saying with this "born-from-above' talk?"
5 Jesus said, "You're not listening. Let me say it again. Unless a person submits to this original creation--the "wind hovering over the water' creation, the invisible moving the visible, a baptism into a new life--it's not possible to enter God's kingdom. 6 When you look at a baby, it's just that: a body you can look at and touch. But the person who takes shape within is formed by something you can't see and touch--the Spirit--and becomes a living spirit.
7 "So don't be so surprised when I tell you that you have to be "born from above'--out of this world, so to speak.
John 3:3-7 (Amplified Bible) 3 Jesus answered him, I assure you, most solemnly I tell you, that unless a person is born again (anew, from above), he cannot ever see (know, be acquainted with, and experience) the kingdom of God.
4 Nicodemus said to Him, How can a man be born when he is old? Can he enter his mother's womb again and be born?
5 Jesus answered, I assure you, most solemnly I tell you, unless a man is born of water and [even] the Spirit, he cannot [ever] enter the kingdom of God.
6 What is born of [from] the flesh is flesh [of the physical is physical]; and what is born of the Spirit is spirit.
7 Marvel not [do not be surprised, astonished] at My telling you, You must all be born anew (from above).
John 3:3-7 (New Living Translation) 3 Jesus replied, "I assure you, unless you are born again, [ a ] you can never see the Kingdom of God."
4 "What do you mean?" exclaimed Nicodemus. "How can an old man go back into his mother's womb and be born again?"
5 Jesus replied, "The truth is, no one can enter the Kingdom of God without being born of water and the Spirit. [ b ] 6 Humans can reproduce only human life, but the Holy Spirit gives new life from heaven. 7 So don't be surprised at my statement that you [ c ] must be born again.
Jesus is saying before you can enter the Kingdom of Heaven you must be born of Holy Spirit, which ties in with the word repent, which means to "turn around your mind" (thinking) we have to first turn around our thinking, before being "Born Again".
Some assume that once they call on the Lord they are saved, is that true, what did Jesus say?
Matthew 10:38 (Amplified Bible) 38 And he who does not take up his cross and follow Me [ [a] cleave steadfastly to Me, conforming wholly to My example in living and, if need be, in dying also] is not worthy of Me.
Matthew 10:38 (New Living Translation) 38 If you refuse to take up your cross and follow me, you are not worthy of being mine.
Matthew 10:38 (New International Version) 38 and anyone who does not take his cross and follow me is not worthy of me.
Matthew 16:24 (Amplified Bible) 24 Then Jesus said to His disciples, If anyone desires to be My disciple, let him deny himself [disregard, lose sight of, and forget himself and his own interests] and take up his cross and follow Me [cleave steadfastly to Me, conform wholly to My example in living and, if need be, in dying, also].
Matthew 19:21 (Amplified Bible) 21 Jesus answered him, If you would be perfect [that is, have that spiritual maturity which accompanies self-sacrificing character], go and sell what you have and give to the poor, and you will have riches in heaven; and come, be My disciple [side with My party and follow Me].
Matthew 7:13-14 (New International Version) 13 "Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. 14 But small is the gate and narrow the road that leads to life, and only a few find it.
To be "Born Again" dosen't happen because you believe in Jesus, being "Born Again" happens when you start living your life the way Jesus did.
It's only by following in his footsteps, walking in the "Way" and start living spiritually, then we are "Born Again"
Then again I could be wrong...