Answer:
This depends. Baptism is practiced in more than one faith, and some in certain rituals. Baptism usually means cleansing, and purification.
In Orthodox Judaism, for example, there is a symbolic bath that women must just prior to marriage.
There are many faiths that have a baptism.
Some involve dunking, and others sprinkling, some in tap water, some in consecrated waters.
Generally you wear a minimal amount of clothing, wearing something that can get wet and not have 'runny' dye. It is likely a white color.
Some religions ordain a special dress code, usually a dress code for babies and children. Shoes shoud not be worn, because they are considered soilded.