Why are you asking only for US and Great Britian when the title states 'countries' instead? Wouldn't it be better to just say it at the title?

Natalia was close, but I'd say

Hi everyone. I am very interested in learning about other cultures. If you are from the US(A) or (Great) Britain, please tell me/us about your culture.

Ok, what specifically are you curious about? Narrow the topic a little and I'll give you a few thoughts.

you can become you  countriy like USA

Hi, everyone. people) I am very interested in learning another culture of the countries. If you are from USA or Britain, please tell me us about your the culture your country)

I think it sounds more natural :)