Teaching pronunciation is a speciality. Look for a qualified teacher with training in linguistics/phonetics. As Peachey said, they should be able to show you what the mouth should do for each sound. A common training sequence is to accurately hear the sounds (phonemes) of English (about 44 distinct sounds with variations depending on the dialect), to produce the sound in isolation, to produce the sound in common words, to produce the sound when reading a text, and finally to produce the sound during conversation. Additionally, English uses changes in tone to show a statement, a question, or emphasis. Again, a qualified teacher can help.