The word "Christian" simply means "like Christ". People who talk and live like Jesus are distinguished by their love and holiness as His followers. But this only describes how one behaves after becoming a Christian. It doesn't tell what makes people like Christ in the first place.
Nobody is really born acting like Jesus. In fact, God says the very opposite. As Adam's children, we inherit his fallen nature. Romans 5:12 says "by one man sin entered into the world, and . . . all have sinned". We aren't sinners because we commit sins - we commit sins because we were born sinners from the start.
Some try to change themselves using will power. With discipline, one can control his behavior and act outwardly better than he really feels. But this only hides the problem. True goodness is not just skin deep - it arises from a righteous heart and overflows into a person's lifestyle. It cannot be disciplined from without. A person who is truly holy does not require heroic discipline just to behave - goodness defines his nature.
So how does someone born naturally sinful make this change? He must be reborn with a righteous nature. This, of course, requires a miracle, but God gives this new birth to all who receive Christ by faith in His name. John 1:12-13 says, "But as many as received him, to them gave the power to become the sons of God, even to them that believe on his name: Which were born not . . . of man, but of God".
Anyone who turns from sin and offers God the sacrifice Jesus made to pay for those sins is born again. He becomes a new person inside, and this changes his life without. What about you? Has God transformed your life through the miracle of the new birth? Would you like Him to? Then simply pray from your heart something like this: "God, be merciful to me, a sinner. I believe Jesus paid for my sins on the cross, and was buried and rose again, and I offer you His sacrifice for my pardon. I repent of my sins, and I accept Your forgiveness by faith. I trust your grace to deliver my soul in the Day of Judgment. Amen!"