Choose an online dictionary that has a choice of pronunciations. Many international dictionaries have two speaker icons that you can click on - one marked US or N.Am, and one marked UK or GB. This will help you distinguish between the British and American pronunciations.

But - as we have told you and other learners many times - the regional pronunciation differences are not nearly as great as outsiders believe them to be. The majority of words are pronounced in the same, or nearly the same way, in British and American English. And whichever way pronunciation you use, it will be understood by any English speaker.

I agree with Jmat that it is not good advice to focus on only one accent. You don't need to choose a native accent. Learners who try too obsessively to imitate distinctive features of American or British pronunciation often add exaggerated sounds to their existing non-native accents and end up sounding strange and unclear. Most learners will speak English with their own accents - a French person will speak English with a French accent and a Vietnamese person will speak English with a Vietnamese accent. And that's fine. The main thing is that what you say is clear and that people understand you.

I disagree that you should focus on only one accent. I suggest you expose yourself to as many accents as possible.

If you look up a word on Forvo (www.forvo.com), you can see where the people who made the recordings are from on a world map. 

The accent clearest to you is the one you're most familiar with. A posh RP accent is widely regarded as a clear, educated accent, but not many people actually speak like that. Because the US film industry is so big, most people are familiar with the standard US accent spoken in movies. But again, I've never met an actual American who speaks just like that.

You could look at the Transatlantic Accent. It's a mix between US and British English and it used to be the standard accent used on TV, but for whatever reason it's fallen out of use. I'm not sure if you'll be able to find materials for it, but if you can it might be worthwhile looking into.

Or you could also learn to speak with an Australian accent!

A website like <em>forvo.com</em> will tell you where the speaker comes from, when you search for the pronunciation of a word. It's easy to compare accents this way.

Another way to focus on one accent is to get feedback from a native speaker. We can easily tell you what your accent sounds like! :)