I just found out I have an 18 month old child and I want the child's last name to be mine and the mother is OK with it. I don't want to have to pay over $300 to change it. Since the father was unknown at birth and never signed the certificate do I just go sign it at the court house or how does this work? Is there a time after I found out?

You should be able to file the paperwork at the court yourself. There will be a fee - its $90 in Florida where I live.

You and the mother go to court, fill out some paperwork. You may spend a few minutes with the judge explaining why you want the name change.

You cannot just sign the birth certificate. That is a legal document and in order to change it once it has been filed requires a court order.