Baptism is an act of obedience in which a believer proclaims externally an internal relationship that already exists.  We believe that the biblical mode of baptism is as a believer, by immersion.

We do it because Jesus modeled it

At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan.  Just as Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove.  And a voice came from heaven: "You are my Son, whom I love; with you I am well pleased."  Mark 1:9-11

We do it because Jesus commanded it

Therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit... Matthew 28:19

It is public, it is for believers, and it is by immersion

Peter replied, "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins..."  Acts 2:38a

Or do you not know that all of us who were baptized into Christ Jesus were baptized into his death?  We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of God the father, we too may have new life.  Romans 6:3-4