Exactly what is being "Born Again?"

Jesus said - "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit" (John 3:6).
So we see it is a birth of our spirit that dwells within.

Is it necessary to be born again?

Jesus said - "Marvel not that I said unto thee, Ye must be born again" (John 3:7).
"...Except a man be born again, he cannot see the Kingdom of God"(John 3:3).

Why is it necessary to be born again?

God said - "For all have sinned, and come short of the glory of God" (Romans 3:23).
"As it is written, there is none righteous, no,not one" (Romans 3:10).
"For there is not a just man upon the earth, that doeth good and sinneth not" (Ecclesiastes 7:20)
It is necessary to be born again because we are all sinners by nature.
"Wherefore, as by one man sin entered into the world, and death by sin, and so death passed upon all men, for all have sinned" (Romans 5:12).

Do we need to repent of our 'sin nature?

"God... commandeth all man everywhere to repent" (Acts 17:30).
Our sins offend God, we need to be forgiven. Because of our sins we are responsible for the death of Christ.
"Who his own self bore our sins in his own body on the tree..." (I Peter 2:24).
"For he hath made him, who knew no sin, to be sin for us, that we might be made the righteousness of God in him" (2 Corinthians 5:21).
Ask his forgiveness, it is a wonderful feeling to be forgiven.

How do we become born again?

After you have repented, ask him to save you and you will be born again.
"For whosoever shall call upon the name of the Lord shall be saved" (Romans 10:13).
"For with the heart man believed unto righteousness; and with the mouth confession is made unto salvation" (Romans 10:10).

We see from God's word we are not born again through baptism, or church membership, or confirmation, or communion, or trying to live by the Ten Commandments.

For God has said - "Not by works of righteousness which we have done, but according to his mercy he saved us..." (Titus 3:5).
"For by grace are ye saved through faith; and that not of yourselves, it is a gift of God - not of works, lest any man should boast" (Ephesians 2:8,9).