"Ça va bien" (pronounced "sa va bee-en") which means "I'm doing well."
However, there are other appropriate ways to respond depending on the context and your relationship with the person asking:
* "Bien, merci" (bee-en mer-see) - "Good, thank you"
* "Pas mal" (pa mal) - "Not bad"
* "Comme ci, comme ça" (kom see, kom sa) - "So-so"
* "Ça roule" (sa rool) - "It's rolling" (informal, meaning things are going well)
* "Et toi?" (eh twa) - "And you?" (to reciprocate the question)
You can also add a bit more detail to your response, like:
* "Ça va bien, merci, et toi?" (I'm doing well, thank you, and you?)
* "Pas mal, j'ai beaucoup de travail en ce moment" (Not bad, I have a lot of work at the moment)
Remember to always be polite and engaging when responding to "ça va," even if you're having a bad day!