美国人说Thank you，该回答什么？

完全可以说sure。

1.Sure
2.You bet
3.You are welcome
4.Not at all
5.Don't mention it
6.That's all right
6.No problem

不用谢。
You're welcome.
不用谢。
You are welcome.
不用谢。
It was my pleasure.
谢谢。不用谢。
Thanks a lot.Don't mention it.
谢谢。不用谢。
Thank you. Not at all.
谢谢。不用谢。
Thank you. You are welcome.
那没什么，不用谢。
That's all right,you're welcome.
算不了什么，不用谢
Forget it
不用谢，我不要任何报酬，不用记在心上。
No thanks,I don't want any payment - skip it.
我知道你很感激，不过那没什么，不用谢。
I know you're grateful,but that's all right,any time

You are welcome / It's my pleasure./ My Pleasure / The Pleasure is all mine. / Not at all./ Don't mention it. / That's OK/ At your service..............等等

最简单的就是
That's ok
you're welcome

that is ok
you are welcome
that is all right
not at al