There is no baptism in the Old Testament, but there are two Old Testament symbols that get wrapped up in the New Testament symbol of baptism. These are baptism and ceremonial washing. These were very important to the Jewish people. If you wanted to become Jewish when you were not born Jewish, you could. You had to be circumcised and you had to be washed clean. Understanding those symbol brings light to the meaning of baptism.
A few days after birth, a male child would be brought to the temple to be circumcised. To understand this moment, you have to understand the biblical language of covenant. You did not make or write a covenant. You cut a covenant. Normally you would cut an animal in half and each take part as a sign of the covenant.
In circumcision, the covenant between the child and God was more literally cut into the skin. This marked physically who that child was and to whom they belong. Namely, they were part of the chosen people and belonged to God. They were part of the covenant and part of the promises God gave to Abraham, Jacob, Moses, and David.
Ceremonial washing was a means for spiritually cleansing after a person was made dirty. Examples of needing to wash in the Old Testament included when a person touched a dead body, when a sick person got better, and when a woman went through their menstruation cycle. In these cases, and numerous others, people had to wash before they could enter the temple again
But where would this washing take place? There is not a lot of water in Jerusalem. It would have to be in a river, or more likely it would be done in one of many washing pools around Jerusalem. These pools had steps that would go down and then back up, so that you could walk the steps and be fully submerged in the middle.
These two symbols do not carry on into the New Testament for Christians. Instead, their meaning is swallowed up into the imagery of baptism. Baptism is a spiritual mark that is cut into a person much like circumcision. It is a sign of acceptance into the community and the place as a child of God. Baptism cleanses of evil and sin and washed the person.