American curriculum or British curriculum?

I don’t know anything about English, and I want to ask you something.
When I watch American movies or series, I understand them easily and find the accent clear and simple.
But when I watch British movies or series, I find them hard to understand.
I found out that there are two English curriculums — American and British.
My friend told me to study British grammar and curriculum, but to watch American content and speak in the American accent.
I’m not sure if that’s correct. Should I follow only one path, or is it okay to mix both? What do you think?
And do you recommend that I study the American curriculum since I understand American programs and content more easily?

I understand what you mean but British accent is better than American accent, because British accent is pure English but I don't know about American accent.