You do not become a Christian by anything you do. You do not become a Christian by being a member of a church. You do not become a Christian by being sincere. You do not become a Christian by anything you say.
What must you do? You must believe in Jesus alone; you must trust Him alone; you must accept Jesus' sacrifice for your sins alone and nothing that you do in any way.
Though there are no rules or steps to becoming a Christian, the following is a biblical summarization of what you need to know and do:
You must recognize your sinfulness before God:
"Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned," (Rom. 5:12)
You must recognize that your sin has caused a separation between you and God:
"But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear," (Isaiah 59:2).
You must believe that Jesus is the only way to God:
"I am the way, the truth, and the Life, and no one comes to the Father but by Me," (John 14:6).
You must ask Jesus to forgive you of your sins because Jesus has the authority to forgive you of your sins:
"Then Jesus came to them and said, ‘All authority in heaven and on earth has been given to me,'" (Matt. 28:18, NIV) ... and
Jesus told you to ask Him for your requests: "You may ask me for anything in my name, and I will do it," (John 14:14, NIV) ... and
Jesus is the one who forgives sins: "But that you may know that the Son of Man has authority on earth to forgive sins....'He said to the paralytic, "I tell you, get up, take your mat and go home,"'" (Mark 2:10-11, NIV).
This last "step" is called receiving Jesus.John 1:12says, "Yet to all who received him, to those who believed in his name, he gave the right to become children of God.
You must turn away from your sin:
"In the past God overlooked such ignorance, but now he commands all people everywhere to repent," (Acts 17:30)
"Produce fruit in keeping with repentance," (Matt. 3:8)
If you want to become a Christian you must accept Jesus as your Savior. You must pray to Him and ask Him to forgive you of your sins and come into your heart.
He will. He loves you. Salvation is only a prayer of faith away.