You've asked two different questions! For the second question, just focus on standard English. I know we like to point out the differences between US and UK English, but the reality is that, in general, we understand each other perfectly well. It's still one language. You could try to imitate what you hear in order to get the intonation, or you could get help from (or have a lesson with) a native English speaker. Some of the teachers here can help with your pronunciation and accent, which includes your intonation. You could get some very useful advice in a matter of minutes, instead of watching an hour or so of video and only guessing. :)

If you want to improve your intonation you'll simply have to practice. I suggest watching videos in English on YouTube (i.e gaming, entertainment, etc) and see if you can try to match the way they speak. You could find a language partner over Skype and try to have conversations to improve - intonation is something that develops with time as you learn the language - there's no quick and easy way of doing it.

You could ask someone if your accent sounds more British or American. I don't recommend changing your accent from one to the other for the sake of improving intonation. In any case, patterns of intonation are broadly similar across all native English-speaking countries. The more important choice for you is which accent and materials you listen to for the purposes of imitation. I recommend listening to one accent consistently. You should over time be able to identify the same patterns when you hear other accents.