How to get custody of my baby. Mom signed custody of child when she gave birth while in custody. I'm not on birth certificate and I had no knowledge of this birth. Mom says they tried to contact me but they couldn't locate me

You will have to go through the family court system where you will be required to take a paternity test.

If I understand you, the mother was in jail when the baby was born. You were not listed on the birth certificate. The mother has signed over custody to someone else.

The first question is when was the child born, how old is s/he? Second question is was the child adopted or just placed in guardianship or in the foster care system? If placed in guardianship is the child with a relative? If adopted, how long ago did the adoption occur and was it with a relative or a third party? The answers to those questions will suggest the probability of your getting custody.

As to how, you start with the local Family Services agency or Family Courts. I would strongly recommend hiring an attorney to help you find your way through the bureaucracy.

Your first task is to prove you are the father of this child and have standing to attempt to gain custody. I highly recommend you hire or at least consult with a family law attorney to help you navigate this bureaucratic legal mess.