If someone says to you "How do you do?" , say "I`m fine, thank you."
Or this: "I`m fine, thanks. And you?"
Dippit is correct that the phrase "How do you do?" is formal, and something you`re much more likely to hear a Brit say than an American. I don`t remember anyone EVER asking me "How do you do?". I`m used to hearing something like "How ya doin`?" or "How`s it goin`?". But, I`m an American, and we`re less formal than Brits the majority of the time.
