How do you do? vs How are you?

We often don't know what to say when someone asks you "How do you do?" and yes often say "fine, thanks!". But that's not right... let's see why!
How do you do?
This is not a question. It is another formal way of saying "Hello." It is also very British less than American English.
The correct response is; "Pleased to meet you." or "How do you do." or just "Hello."
We only really use it the first time we meet someone.

How are you?
This is a question that refers about your state, or humor, etc.
A response is; "I'm fine thanks. And you?"