How can I speak like an American or British native speaker?

My advice is to listen to more TV shows, radio broadcast and American movies. Then meet with American speakers and listen, observe ... and then participate with asking for feedback when you're ready for it.

Idiom--in many cases the regional differences in English boils down to idiom. As Churchill said, 'America and England are separated by a common language.'

American TV boils down to a midwestern or a California accent and the best accent models (free of idiom) is newscasters. On the other hand many professional Comics use idiom as a basis of many jokes.

Rap