You will need to attempt to find the father, maybe asking friends, family?
If you cannot, then you need to serve notice of the adoption by publication, meaning put a notice in the newspaper of your city. Contact
Child Adoption | Step Parent Adoption | Domestic Adoption
I used them, and they're great. They can give you specific rules to follow to insure the adoption is done correctly, especially when the bio dad is missing, plus they'll complete your adoption paperwork for you.
Good Luck,
Pamela