Baptism is a way of showing others that you have entered into a relationship with Jesus Christ. It symbolizes the burial and resurrection of Jesus Christ, illustrated by going under the water (burial) and raising up out of the water (resurrection). While at the same time, it also symbolizes your new life as a Christian, by going under the water (dead to your old life), then raising up out of the water (living a new life). The Bible mentions this symbolism in Colossians 2:12 and Romans 6:4. Baptism doesn't make you a Believer, it demonstrates that you already believe. It's important to understand that baptism does not "save" you. You are saved only by your faith in Jesus.
For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast. Ephesians 2:8-9
Baptism is like a wedding ring -- it's the outward symbol of the commitment you make in your heart.