One of the most effective ways of improving English pronunciation is using syllable stress accurately.
In English only one syllable, in words with two or more syllables, is stressed. The stressed syllable is pronounced with a higher pitch, louder volume and a longer vowel. In contrast, the vowels in the other unstressed syllables are reduced - said more softly, in a lower pitch and pronounced less precisely.
If a word is stressed incorrectly the speaker can be very difficult to understand.
When unsure of which syllable to stress in a particular word using an online dictionary, such a dictionary.com, can be helpful. Not only will the stressed syllable be indicated, you can also hear the correct pronunciation of the word.