For Catholics: Contact the parish where you were baptized, and if you were baptized in another country, the diocese, and they should be able to send you updated certificates.
For those baptised in a Non-Catholic tradition: If you were baptized in a denomination other than the Catholic Church, it is possible that you will have some difficulty obtaining certificates.
In either case, whether Catholic or not, if you are unable to obtain a baptismal certificate, you may print out a copy of the following testimonial letter and have it signed and sealed either before a preist or a public notary, and this will serve as proof of valid baptism.