I am very glad to have helped people by publicising the plenary indulgence that is offered on the anniversary of one's baptism. Some people are not sure of the date of their Baptism and might like to find out. Here is some advice on how to do so.
Every parish keeps a permanent register of Baptisms. This is one of the most important things that the parish possesses. If things are done properly, your confirmation should also be recorded and also your marriage (or religious profession, or ordination.)
A "Baptism Certificate" is a copy of the entry that is made in the Baptismal register. If you are getting married, you need to supply a recent baptismal certificate (within six months of the wedding) which will then also record that there are "no other entries" (i.e. no previous marriage recorded) and that you are "free to marry". I always send for these myself but some priests ask the Catholic party(ies) in the wedding to obtain them. It is also OK to ask for a Baptismal certificate if you just want to know when your baptism was.
If you want to get a certificate of Baptism, send a letter addressed to the parish priest of the parish where you were baptised. Give your full name (as it was at the time of your baptism if your name has subsequently changed.) Give your date of birth and roughly when the Baptism took place. If you were baptised as a baby it is sufficient to say this but if you were baptised at three or four years old, it will help the parish priest if you tell him this; and enclose a stamped addressed envelope just to make things a little easier!
My own practice now is to photocopy the entry onto parish letterhead and then sign date and stamp it.