Before you can learn to speak, you must learn to listen. This is basically just you making a conscious effort to not just hear what someone with the pronunciation you want is saying, but to listen to exactly how he pronounces it. Many words in English are spelt differently but pronounced the same so listening will also help you identify which word is being used as per context. It gives you a mental image of what each word is supposed to sound like so that you start speaking, you know whether you're getting it right or wrong.
Then the next thing to do is to learn how to shape your mouth. To pronounce a word correctly, you must learn to shape your mouth and position your tongue the right way. A slight difference in this can produce an entirely different pronunciation and so you must practice well.
If possible, find a partner who will help you learn and practice with you. You can both be learning or he can be good at it but either way, having a friend will make learning easier