Unlike other world religions, which tell people to be good by doing good works, and work your way to heaven, Christianity does not teach the idea of becoming good by doing good works. Christianity deals with the heart. If your heart is not changed, it doesn't matter how many good works you do, the internal rottenness will never go away.
The Bible gives us the correct diagnosis of the problem in Jeremiah 17:9-10: "The heart is deceitful above all things and beyond cure. Who can understand it? "I the LORD search the heart and examine the mind, to reward each person according to their conduct, according to what their deeds deserve."
The Bible tells us to change from the inside out. In the New Testament Jesus said to the Pharisees: “Woe to you, teachers of the law and Pharisees, you hypocrites! You are like whitewashed tombs, which look beautiful on the outside but on the inside are full of the bones of the dead and everything unclean. In the same way, on the outside, you appear to people as righteous but on the inside, you are full of hypocrisy and wickedness." (Matthew 23:27-28)
Often, Christians can make the mistake of trying to get closer to Jesus by doing good works. We always need to remember that Christianity is not a religion of rules to follow - of a list of do's and don'ts - but rather Christianity is a relationship with Christ. There is no Christianity without Christ. So instead of focusing on doing good we need to focus on Jesus. Consider that he promised that he would be with us always. When we follow Christ with all our heart, and walk by faith, good works will follow. Then we will not want to do anything that displeases Jesus.
Now having said that, it does not mean that we do not sin after becoming Christians, rather we become more sensitive to the sin. Since we are still in the flesh, and the spirit and flesh are fighting with each other, we sometimes fall into sin. But as I said before, we Christians become sensitive to sin and if there is any sin in the life of the believer, it will bother that person and it needs to be immediately acknowledged and we should repent of it then the grace of God covers us and the blood of Jesus cleanses us from all the sin. As the Bible says in 1 John: "My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father—Jesus Christ, the Righteous One."
When we believe in Jesus Christ, we accept him as our Lord and Savior. Sometimes people gladly accept Jesus as their saviour but it is very difficult to accept and live as he is the Lord. We all struggle with it. Anything that keeps you from putting Jesus first in your life, get rid of that. Whether it's a habit, lifestyle, material things or people, we should distance ourselves from it which is not easy for us. We can do it only with the grace of God which comes through Jesus Christ. As the Bible says in the Gospel of John 1:17: "For the law was given through Moses; grace and truth came through Jesus Christ." It's the grace of God that lets us live a Christian life under the lordship of Christ Jesus. We need to keep examining ourselves according to his word. That is why we need to grow as a Christian. Not only reading God's Word but studying and meditating on it and walking in obedience to it in our lives.