Depending on how you look at it, there could be two ways to answer this question.
If you look at it from a theological point of view, removed of human passions and emotions, focused solely on what is happening at the moment of conversion, then you might answer as follows:
"Nothing. A person can't actually do anything to be saved. God causes a person to be born again. Nothing a person does can contribute to that. Even faith is a gift from God."
If you look at it from a more personal, front-line, emotional, practical, point of view, then you might say something like this:
"If you have heard the gospel, the proper response is to believe it in faith and to turn away from doing and thinking the things which you know are wrong. You must now live in the way that you know Jesus would want you to live. Copy him and live for him."
These are both correct responses, but we can get caught up trying to be either too practical or too theological. We must remember that a new believer will likely struggle with the deep theological significance of what is happening to them. For them, they need a simple call to action as Jesus demonstrated:
"Jesus came into Galilee, proclaiming the gospel of God, and saying, “The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.” " - Mark 1:14-15
Yet at other times, a theological focus is needed, as Paul demonstrates with his letter to the Ephesians:
"For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. " - Ephesians 2:8-9
As always, what has actually happened is that a person has been born again by the power of the Holy Spirit. Their faith and actions of repentance are the fruit of the new creation that they have become. Yet it is a truth that Christians need to be called to both faith and repentance, at times. This is an important role of the community of believers in building one another up in Christ, to his glory.
