How does someone become a Christian? By entering into a personal relationship with God. It's in the Bible, John 17:3, TLB. "And this is the way to have eternal life—by knowing You, the only true God, and Jesus Christ, the one You sent to earth!"
Becoming a Christian is changing in response to the gospel message. It's in the Bible, Acts 2:37-38, TLB. "These words of Peter's moved them deeply, and they said to him and to the other apostles, 'Brothers, what should we do?' And Peter replied, 'Each one of you must turn from sin, return to God, and be baptized in the name of Jesus Christ for the forgiveness of your sins; then you also shall receive this gift, the Holy Spirit.' "
Becoming a Christian is personal and public. It's in the Bible, Romans 10:8-10, TLB. "For salvation that comes from trusting Christ—which is what we preach—is already within easy reach of each of us; in fact, it is as near as our own hearts and mouths. For if you tell others with your own mouth that Jesus Christ is your Lord, and believe in your own heart that God has raised Him from the dead, you will be saved. For it is by believing in his heart that a man becomes right with God; and with his mouth he tells others of his faith, confirming his salvation."
It is important that one understand that "believing" in Christ is more than an opinion, You need the Holy Spirit to live the Christian life. It's in the Bible, Romans 8:9, NIV. "You, however, are controlled not by the sinful nature but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ."
Christianity encourages honest investigation. It's in the Bible, Luke 1:3-4, NIV. "Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, so that you may know the certainty of the things you have been taught."