Baptism is a way of showing others that you have entered into a personal relationship with Jesus Christ. It symbolizes what took place in your heart at the time of salvation:
You accepted Jesus Christ as Lord and Savior.
You shared in his death and resurrection, illustrated through baptism by going under the water (dead to your old life), then rising again (new life).
You were symbolically washed clean, and your sins were forgiven by his death on the cross.
Because Jesus instructed us to be baptized, baptism is an act of obedience. While it is not necessary for salvation, it demonstrates submission to God. We immerse because we believe Scripture shows that Jesus modeled that for us as the way to publicly acknowledge our faith in him.