The New Testament Greek word that translates 'baptize' is 'baptizo,' meaning literally to dip, submerge, or immerse into something. We see it first in the ministry of John the Baptist, who was baptizing people in the Jordan River (Mark 1:5) because there was 'much water there'. Later we see Phillip taking the Ethiopian Eunuch 'down into the water' to baptize him. Baptizing or Immersing in water is a graphic illustration of and identification with the death, burial, and resurrection of Jesus Christ.
"Therefore we have been buried with Him through baptism into death, so that as Christ was raised from the dead through the glory of the Father, so we too might walk in newness of life."
Romans 6:4
Baptism is a necessary directive Jesus issued to His followers in the Great Commission. It serves as a public testimony of personal repentance and faith in Jesus and an affirmation of the Triune nature of the One true God.
"Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit"
Matthew 28:19
Anyone who wholeheartedly turns from their sins and trusts Jesus Christ to be their Lord and Savior can be baptized. The criterion of faith in Christ as a qualification necessarily preceding baptism can be seen in the conversation between Phillip the deacon/evangelist and the Ethiopian eunuch.
Then Philip opened his mouth, and beginning from this Scripture; he preached Jesus to him. As they went along the road, they came to some water; and the eunuch said, "Look! Water! What prevents me from being baptized?" And Philip said, "If you believe with all your heart, you may." And he answered and said, "I believe that Jesus Christ is the Son of God." And he ordered the chariot to stop, and they both went down into the water, Philip as well as the eunuch, and he baptized him."
Acts 8:35–38
The biblical order of belief in Christ preceding baptism is also seen repeatedly throughout the book of Acts.
Acts 2:27-38, 40-41, 8:12-13, 9:17-18, 10:44-48, 16:14-15, 18:8, 19:5, 22:14-16
Baptism is a public profession of an individual's faith in Christ. It speaks of God's divine nature, Christ's atoning work, and our personal belief in and commitment to the Gospel.