Go to the church where you're baptized and provide your name, birthdate, parents' names, and the date when you're baptized. This way the church staff can easily find you in their records.
Can you get the baptismal certificate in other Catholic Churches?
No. Only the church where you're baptized has your records. For example, I got mine from Saint Michael the Archangel Parish (San Miguel, Tarlac City) and my girlfriend got hers from San Sebastian Cathedral (F. Tanedo Tarlac City). Payment can be in the form of donation or a required amount with receipt (ours is P100 each). You can claim the certificate either same day or after a week (it depends).
What if you don't know your baptismal date?
The job gets easier if you know when you're baptized. The church staff can instantly find you in their records. If you don't know, ask your parents the approximate date (month and year) of your baptismal so the church staff will know where to start.
Baptismal certificate for church wedding
Make sure the certificate has indicated "
For Marriage Purposes" or something like that. This is a must as a Catholic church wedding requirement.