Once you pray to God, confessing your sins, and calling for Christ to save you, he will. He will forgive you of your sins, and you instantly become a child of God. You enter into a permanent relationship with him, that will last forever. You don't have to worry about Hell and judgement anymore, and you can look forward to spending eternity, with Christ, in Heaven.
However, now we have to go back to what it says in 1st John 1:5-7. This is the passage of scripture which comes right before the earlier passage which was mentioned, in 1st John 1:8-9.
1st John 1:5-7, reads as follows:
Now the message that we have heard from his Son and announce is this: God is light, and there is no darkness at all in him. If, then, we say that we have fellowship with him, yet at the same time live in the darkness, we are lying both in our words and in our actions. But if we live in the light--just as he is in the light--then we have fellowship with one another, and the blood of Jesus, his Son, purifies us from every sin.
-1st John 1:5-7
This portion of scripture reminds us of the change that takes place, when this prayer is said with complete sincerity. This isn't just some prayer, whose words are magical. It's all about the legitimate choice that you make, in your heart. It's a choice to follow Christ, which means turning away from a life of sin. We can know that our faith is real, when we witness this change in our own behavior. Sincerity is key.
Now does this mean that we never sin again? No. After all, we're still not perfect. We will still struggle with sin. However, we consistently turn to Christ --praying to him and the Holy Spirit for help, so that we will obey him, rather than sin. You've chosen to be on God's side, in a spiritual war.
Whenever you are tempted to sin, and should you ever fail to obey God, turn back to these portions of scripture; particularly 1st John. Call out to God and remind yourself that you must always be honest with God about your sin. Be quick to confess it, turn away from it, and ask God to change you. Always be sincere.
So, that's it. Becoming a Christian is a choice. It's when you confess your sin, and call out to Christ, for salvation, with all sincerity. It's a choice to make him Lord of your life, and to follow him, by turning away from your sinful ways.
Don't be fooled into thinking that it's more complicated than this.
I, often, like to think of one of the most revealing conversions that I can think of. It took place, right beside Jesus, when he was dying on the cross. This account is recorded in Luke 23:32-43. It reads as follows:
Two other men, both of them criminals, were also led out to be put to death with Jesus. When they came to the place called "The Skull," they crucified Jesus there, and the two criminals, one on his right and the other on his left. Jesus said, "Forgive them, Father! They don't know what they are doing."
They divided his clothes among themselves by throwing dice. The people stood there watching while the Jewish leaders made fun of him: "He saved others; let him save himself if he is the Messiah whom God has chosen!"
The soldiers also made fun of him: they came up to him and offered him cheap wine, and said, "Save yourself if you are the king of the Jews!"
Above him were written these words: "This is the King of the Jews."
One of the criminals hanging there hurled insults at him: "Aren't you the Messiah? Save yourself and us!"
The other one, however, rebuked him, saying, "Don't you fear God? You received the same sentence he did. Ours, however, is only right, because we are getting what we deserve for what we did; but he has done no wrong." And he said to Jesus, "Remember me, Jesus, when you come as King!"
Jesus said to him, "I promise you that today you will be in Paradise with me."
-Luke 23:32-43