Baptism is going public about your faith in Jesus and communicating to the world your heart-felt commitment to following Him. This step of faith is done after making a personal decision to accept Jesus as your Lord.
Baptism itself does not make us a believer in Christ, but rather it is a way of saying to the world that our relationship with Jesus, and our faith in Him, has already begun. In the same way that Jesus died, was buried and rose to new life, baptism symbolizes that our old self is gone (going under the water) and our new self has come (coming above the water). We have been raised to walk in a new life with Christ.