Baptism is an ordinance instituted by Jesus Christ in the New Testament (Matt 28:19) for those who confess faith in him (Acts 2:38; Mark 16:16; Acts 8:12). Baptism is done by immersing the whole body in water (Matt 3:16; Acts 8:38,39), in the name of the Father, and of the Son, and of the Holy Spirit (Matt. 28:19).
When you are baptized, it serves as a sign of your fellowship with Christ and your identification with him in his death, burial, and resurrection (Rom 6:3,5). Baptism marks that you have now put on Christ (Gal 3:27) and received forgiveness for your sins (Acts 22:16). It is the public declaration of your repentance and your belief in Christ (Acts 2:38), symbolizing that you have committed to live and walk in newness of life through Christ (Rom 6:4).
When you decide to be baptized, you are acting in obedience with Christ's command in Matt 28:19,20. At the moment you decided to place your faith in Christ, you were saved (Acts 2:21; 4:12). When you are baptized in water, you are acting out, in the physical realm, what has already happened to you in the spiritual realm, your death, burial, and resurrection through Christ (Gal 2:20; Rom 6:3,5; Col 2:12). Baptism by immersion best illustrates the symbolism of our shared death (crucifixion), burial, and resurrection with Christ. Going into the water symbolizes death and burial. Coming up out of the water symbolizes being raised with Christ to walk in newness of life.