Baptism is an outward sign of an inward change. We know that when Jesus died on the cross, He wiped away our sins and made us clean. Baptism is our way of showing others that we have been wiped clean of our sins. Baptism happens AFTER we believe Jesus is who He said He is, and we have given our lives to follow Jesus. Your heart changes first; then you are baptized to symbolize the change in your heart.
Baptism symbolizes our death to sin and new life in Christ.
Being put into the water represents the death of the "old self" with our sinful way of life. Being completely covered by water symbolizes burial, and being raised from the water pictures a resurrection to "newness of life." After baptism we consider ourselves dead to sin, no longer choose to live a sinful way of life. Rather, we give our lives to God and use our time to become an instrument for good - to mature in our thought life and in our actions.
Baptism is an act of obedience to follow what God told us to do and to follow Jesus' example.