contact the archivist or the head of the liturgy and sacraments office of your diocese if the parish where you were baptized has closed
gather all the relevant information you can: your name, date and place of birth, parents' names (including mother's maiden name), godparent names if you know them, date, city, church of baptism, the more info you have the faster someone can help you. Don't leave messages with someone who does not have the job of locating records, keep calling until you talk to that person, better yet, get their email
only you, if you are an adult, or the parents of a minor can get your record.
It may be much faster and more direct to ask the parish where you are getting married, confirmed etc. to request the record, as it can be sent directly to another parish pastor.
If your parents were in the military, or migrants, or if you were born and baptized in another country it may be harder to find. For the first two, the record may be in the corresponding office in Washington DC.
almost every diocese also creates preservation microfilm of all sacramental records periodically so there may be a way to search that way.