How are you?

This is a question for you americans out there:

How am I supposed to answer to this question?

I live in europe and a lot of Americans come to the hotel I work at. And every time anyone asks me 'How are you?' it feels like my answer is wrong.

If I reply with 'I am good, how are you?' some people look at me weird.

If I just say 'How are you?' back then some don't even answer and others look at me weird.

I don't know what to say! Some americans answer normally and other just look like they just saw a ghost.

Can someone help me out? What is the right answer when an american asks 'how are you?'