Baptism is an outward expression of what you have already decided in your heart.
Water baptism is a public declaration of three very important things:
You have accepted Jesus Christ as your personal savior.  “Those who accepted his message were baptised” Acts 2:41
You are beginning a changed life in Christ.  “We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life.” Romans 6:4
You are part of a new family.  “So in Christ Jesus you are all children of God through faith, for all of you who were baptized into Christ have clothed yourselves with Christ”  Galatians 3:26,27