One way to improve pronunciation is to simply listen to people speaking English. This will help you hear the ways words are spoken. Then, you can compare how you say them and learn from that. While doing this, it is important to take into account that some places, English is spoken with an accent that varies from region to region, but it is still very helpful to listen to people talking.
The alphabet has regular letters, but in the front of most dictionaries there is an alternative alphabet that is accompanied by a legend of the symbols used. This is called the phonetic alphabet. Each symbol represents a different way to say a letter (i.e. a long "a" or a short "a"). After each word listed in the dictionary, it is typed out using the phonetic alphabet. Practice saying the words using this method.
A great way to learn is to simply practice speaking English. Sometimes, when people speak different languages, it is hard for them to say certain letters like "th," "l," and "r." These letters sound completely different in other languages and, therefore, give people a rough time when learning to speak English. Also, by speaking English you will learn where to stress certain words. Stressing the wrong syllable can make a word sound completely different than it was intended to.
Some words in English have similar spellings that are not phonetic, so pay attention to the spelling of words as this will help with pronunciation. For example, "ough" can sound like "uff," as in "tough," or "off" as in "cough." These are important differences to note because they can cause problems when a person tries to say these words. However, others like "tion" will consistently sound like "shun."
Another idea for helping English pronunciation is to make yourself a list of how words sound. There is the phonetic alphabet, but it can help to make your own. For example, if you find it difficult to remember how to say a word like "speak," then write your own phonetic note next to it like "spEEk." This will help you remember which syllable to stress and how to say the word.