It depends on where your child was born and whether it is registered in the Registry of Births. If it was born in Serbia, and registered in the Registry of Births, then you should contact the local Registry Office in the municipality where your child was born. If your child was not born in Serbia, you should then contact the embassy or consulate of the country in which the child was born.
If your child has not been registered in the Registry of Birth in any country, then you should contact free legal aid service office or NGOs which deal with such issues, or you can hire a lawyer.