You have been given the gift of choice, and with that gift you are able to choose to believe God sent his son to take your punishment or reject it.
"That if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved." (Romans 10:9)
The Salvation Prayer is merely a road to rebirth in Jesus Christ. To be born again you must confess Jesus as Lord and believe that He is the Son of God sent to redeem mankind from the bondage and penalty of sin and that he died to take our punishment. When you ask Him into your heart, you are allowing Him to be the Lord of your life.
Prayer of Salvation