Baptism is the first step in obedience to God after you have accept and called on Christ as your personal Savior for paying your sin debt. You're not saved or born again by being baptized. The scriptures is clear of this. The Bible says that the only way to Heaven is by being saved through Christ Jesus by calling out to him (Romans 10:13).
It's showing your testimony of salvation before men that you choose to follow God in your life as John the Baptist did when he baptized.