In Chapter 3 of the Fourth Gospel, Jesus explains to Nicodemus how Men may attain the Heaven, Kingdom of God.
My comments are in italics.
John 3 -  Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
Unless we are born again - that is, die and are resurrected, we cannot dwell in Heaven.
 Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?
 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.
 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
 Marvel not that I said unto thee, Ye must be born again.
To explain what he means by 'born again', Jesus refers back to the comments by John the Baptist in Chapter 1:
 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
 This is he of whom I said, After me cometh a man which is preferred before me: for he was before me.
 And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.
 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.
 And I saw, and bare record that this is the Son of God.
Jesus is explaining what it means to be 'born again' - and makes clear that this refers to being reborn/ resurrected as divine (completion of the 'process' of divination, theosis, sanctification).
John the Baptist is saying that before he met Jesus he was 'merely' baptising with water - so during baptism the spirit would descend and touch - then leave the baptised person; which only affected this life: the spirit affected them but did not make them divine. But when John baptised Jesus, the spirit abode on him, that is - Jesus became divine. John cannot make anyone divine, but Jesus can.
Because Jesus is now divine, he can 'baptise' Men with the Holy Ghost - can make Men divine. But we are told elsewhere in this Gospel that Jesus did not literally baptise anyone (by immersion in water, only his disciples did this), so the implication is that when Jesus 'baptises' it means something not-literal. What it means is that Jesus transforms us to become divine by our own death and resurrection (born again); as Jesus was thus transformed at the baptism by John.
Later in the Gospel, Jesus tells us that this transformation happens simply by us believing the identity and nature of Jesus, by loving him and following him through death to becoming divine in resurrection; as a sheep follows the Good Shepherd.
 The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.
The wind and the spirit are one, Jesus is telling us what it is like to have become divine in contrast to the state of mortal Men; describing poetically the nature of divine experience.
 Nicodemus answered and said unto him, How can these things be?
 Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?
 Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.
 If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?
 And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.
 And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up:
 That whosoever believeth in him should not perish, but have eternal life.
Jesus knows these things because he has already dwelt in Heaven, in the Kingdom of God (implicitly, in spirit, before Jesus was incarnated; when he was co-creating this world with his Father as described at the start of the Fourth Gospel). The Son of man is Jesus after his ascension to Heaven, after being lifted up.
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
 For God sent not his Son into the world to condemn the world; but that the world through him might be saved.
Jesus is emphasising that he brings Good News; and enhancement of the human condition, a new possibility for all Men (the world) - but operative only for those that choose to believe in him.
 He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
Condemned to dying (and not being resurrected, not becoming divine) 'already' because that is the default situation, death is what already happens in the world before Jesus came.
 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
But Jesus prophesies that many Men will not choose life everlasting, not choose to become divine, choose not to dwell in the Kingdom of God...
 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.
Because the evil do not want such things. They prefer death to divinity. To follow Jesus must be a free and loving choice, it is not forced upon anyone.
 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.
To 'do truth' is to believe, love and follow Jesus through death; and be born again into resurrection as a divine, eternal Man - and to dwell in God's Kingdom of Heaven.