Why don't you leave that to God? If they have believed on Jesus Christ and have been born-again, the Holy Spirit inside them will guide them into all truth. The fruit of the Spirit will take fruition in their lives and they will walk in love. What do you mean, "tell them the truth?" Did you tell them the Gospel? Then you told them the truth. If they confessed with their mouth and believed in their heart Jesus Christ, then all is well. Let the rest take care of itself by God working in them, through the process of sanctification.
KennethC brought up the Apostle Paul and his addressing of sin. Realize that Paul dealt with sin in people lives by showing them that they are not acting as who they now are in Christ. They are not acting in alignment with who Christ has made them to be, new creations. People know what is sinful, their conscience bears witness. They need the solution. Its grace, its renewing our minds to who we are in Christ, Jesus.
The idea of repentance being a turning away from sin is more so a boast in the flesh than anything else. True repentance is a turning to Christ and in Jesus Christ we have victory over sin. There is no fleshy boast of a promise to forsake sin, a promise that is empty and can't be kept.
Rather, we go to Christ as sinners and in Him we are transformed into saints. We don't clean ourselves up before taking a bath, in the same way we don't stop sinning before coming to Christ. It is through Jesus, being born-again, having the Holy Spirit residing in us, the fruit of the Spirit and renewing our minds to who we are in Christ that inevitably transforms our outward actions/conduct. It is not a work of our own, but a process of sanctification done by God and through His revelation given to us.
You tell them that they will change as a result of being born-again. Things that they desired once are purged from them. Ways in which they have acted are no longer in alignment with who God has made them to be. They will understand that their actions do no align with who they now are in Christ. That is, the righteousness of God, in Jesus Christ. God will awaken them to righteousness. When people come to Jesus Christ and are converted a change has taken place, and it will have its fruit. Mind you, fruit takes time to bear, to come to full fruition (ripened) and so you don't need to rush the process and condemn babes in Christ. Let them mature at their own pace and as God sees fit.