A good way to practise pronunciation is the BBC Learning English website. Watch the introductory video and try the various pronunciation tips.

If you have problems with pronouncing a certain word, e.g. “definitely”, “miscellaneous”, or “Sioux”, the following website might be of some help: the English Pronouncing Dictionary with Instant Sound.
Simply turn on your speakers, type in the word and listen to how it is pronounced.

Many online dictionaries also offer audio files for each word.

However, there is no "perfect" pronunciation. Watch the following video and learn about the many different varieties (dialects, accents, etc.) of the English language: