My father is from and born in Greece, and I was born here in the US. My parents were never married. Can I get a dual citizenship? I am now over 18. How do I obtain his birth certificate for application?

As long as your father formally recognized you as his legitimate child before you turned 18, you have a claim to citizenship. This formal recognition needs to have been valid and legal in the state where you were born.

If you do have this proof, take it to the Greek Consulate and they will tell you what other information you need.