Learn to listen. Listen to other people and listen to yourself. Pay attention to detail.
Notice how your mouth and lips move. Are they moving like those of an English speaker?
Pay attention to your tongue. Are you doing what an English speaker would do?
Break words down into sounds. What specific sound is not coming out right?
Add stress to sounds and words. Enunciation makes a big difference.
Use pronunciation podcasts and videos.
Record yourself and ... listen! What is wrong? Pinpoint your mistakes. That is the first step to start improving: finding out what it is that you are not doing right.
Practice with somebody (somebody who has a native English accent) who can help you. Preferably, an expert!