Baptism means you have decided to repent of your life apart from God and have decided to go the other direction in your life, to walk with God as your Lord.
Matt 3.6 describes people confessing their sins, repenting, being baptized by John the Baptist in the Jordan river.
Romans 6.4 and Colossians 2.12 say baptism is a symbol saying the believer is buried with Christ (going under the water) and raised to new spiritual life with Christ’s resurrection (coming up out of the water). By identifying with Christ’s burial and resurrection the believer is saying he knows and depends on God considering Christ’s death as paying for his sins, since Christ died and then was buried, and that the believer knows and depends on now having a new life with Christ, being “born again” to this new life, raised from the dead as Christ was.
The Dictionary of Bible Types describes it thusly, “A type of burial wherein the believer accepts God's condemnation of himself, admits that he had to die at Calvary, and therefore should be buried out of sight in a watery grave. Thus he justifies God's diagnosis of his case, and proves it by going through this symbolical burial. Those who refuse to be baptized thereby reject God's testimony about their wickedness and sinfulness. They refuse to admit that they are so bad that they should be put to death and buried.”
Galatians 3.27 says that by being baptized a believer is saying he has clothed, or covered himself with Christ. In effect, he is saying, “My life is hidden in Christ (Colossians 3.3). When you see me I want you to be seeing what Christ is like.”
Baptism means you are telling the world you are a follower of God through Jesus Christ.
Mark 1.9-10 and Luke 3.21 describe how even Jesus Himself was baptized, an example for all other believers.
Ac 8.26-38 describes an official of the Ethiopian government that God saved. God sent Philip to help him understand about salvation. The man believed and immediately wanted to be baptised. Acts 8.38 describes that.