I am faith. I am belief. Except for when I'm not. - Joe Pug
The defining human issue in which Christianity distinguishes itself from other religions is this: how can a person become right with God? While every other religion employs some version of human merit on the basis of moral purity, Christianity rises and falls on the idea of Grace: God makes us right with Him. Instead of us finding our way to God, He comes to us.
The story of Christianity is not a moral story, it is not about how pretty good people get their houses in order. There is no such thing as a moderate salvation story. Either a person needs saving or they don't. Christianity is a story of rescue. The people who need saving? All of us. When the Bible discusses how we need saving it uses the language of 'sin' to describe the way we are not who we were created to be. At the core of pain and suffering, abuse and neglect of all sorts, at the heart of evil wherever it is experienced, is sin. The Bible describes it as the twisting and distorting of the world into crookedness. Humanity experiences it daily in the tendency to do what we do not want to do and the frustration that we cannot do what we ought. Sin is present in our wounding of others and the way we have been wounded too. If we were left alone by God sin would continue to spiral downward into greater suffering until the end of our world. If left alone there would be no hope for the end of pain and evil. But God did not leave us alone.
The symbol at the core of Christianity is the Cross, and this Cross is where Jesus saved us. The Cross is where our inability to save ourselves and God's love for us unsavables met face to face. In the death of Jesus Christ, the Son of God, and in his resurrection, sin was dealt a deadly blow. The story of Christianity is the story of God pushing back against the effects of sin in our world, including in our own hearts. It is the story of God, like a king, impressing his rule on a broken and suffering world. The Church announces the good news of that Kingdom and participates in its healing and restoring work.
When a person "believes" in Jesus they are placing their trust in Him. But that trust is always imperfect: one day we may excel at faith and the next day we may not. Maybe even hour by hour we feel both Christian and non-Christian. The cost the Christian pays is that we must own up to the fact that we cannot save ourselves and we cannot live right lives without dependence on Jesus to make Christians out of us. So the same way you come to Jesus is the same way you live as a Christian: by needing Him every day. This is what Christians call repentance. You could even say that the prayer of salvation is the posture of a Christian's heart from that day forward. There are no pedestals to stand on in Christianity. The Christian never graduates from a basic childlike dependance on Jesus.
The sign that we have trusted Jesus with our lives is not the elimination of all sin or pristine morality. It is what the Bible calls fruit: while sin remains in our lives we also begin to see new life: love for our neighbors and even our enemies, joy and peace, patience and kindness, self-control, compassion and mercy for others. Whatever greater morality a Christian realizes as they follow Jesus is expressed in greater humanity and service toward others. That greater morality is seen in sacrificial love and in the pursuit of justice for the poor and oppressed. And worship. The person who has been given grace by Jesus worships Jesus.
The most important thing you can do to grow in your life as a Christian is to join a local Bible-believing church and worship with other Christians. When the Word of God is proclaimed and the sacraments are given (communion and baptism) Christians are nourished in their faith and the grace that makes them right with God is the same grace that makes them grow.