Here's how you can answer, depending on your situation:
Formal:
* Mi papá se llama [nombre del padre]. (My father's name is [father's name].)
Informal:
* Mi papá se llama [nombre del padre]. (My dad's name is [father's name].)
* Se llama [nombre del padre]. (His name is [father's name].)
If you don't want to share your father's name:
* No puedo decirte. (I can't tell you.)
* Prefiero no decirlo. (I prefer not to say.)
Important Note: It's good to use the formal "usted" when addressing someone you don't know well, or when you're in a more formal setting. However, "tú" is generally used when speaking to friends and family.