Listen carefully to broadcasts, etc. in English, and try to copy the sounds, slowing down as required. The more you practise focussed listening, the better you form the mental 'image' of the sounds that you must then try to reproduce. It helps if you can record and listen to yourself.
I don't think that speaking with people, as Kuro [above] suggests, as would so many others, is necessarily the best way to improve pronunciation. It is awkward for a partner to halt the flow of conversation in order to make corrections, which then need to be rehearsed. It is also more difficult to focus on pronunciation when, at the same time, you are trying to understand what has just been said, and work out what to say next. Until you are fluent, and have a very good command of the language, these activities are likely to interfere with your attempt to concentrate attention on the sounds.
Of course, you do speak to yourself while practising !
I suggest you to follow some teachers on youtube who specifically teach how to pronounce word by word. It's not only fun but also educational and beneficial to improve quickly.