Vietnamese and English have very different pronunciation systems, so you will probably need to practise the individual sounds of English (there are 44 in all.) You can get a book + CD like "Ship or Sheep" and listen and repeat a different sound every day - it's a bit boring, but after a few weeks you will notice an improvement.
The best thing is to practise with a native speaker. If you can't, watching DVDs with subtitles will help, and BBC/ CNN news if you can get it (if you video a TV programme, you can watch it slowly and it will be easier to understand.) You can also practise with songs - a lot of lyrics can be downloaded.
You could also try speaking onto a tape, listening to yourself and comparing it with native speakers. For this, it's really good if you can find a teacher or good speaker to help you. The most important thing is to try to enjoy it - choose films, tapes etc that you can enjoy, and if you get a chance to practise conversation, enjoy the conversation and don't worry about correct grammar (yet.) Good luck!
The best thing is to practise with a native speaker. If you can't, watching DVDs with subtitles will help, and BBC/ CNN news if you can get it (if you video a TV programme, you can watch it slowly and it will be easier to understand.) You can also practise with songs - a lot of lyrics can be downloaded.
You could also try speaking onto a tape, listening to yourself and comparing it with native speakers. For this, it's really good if you can find a teacher or good speaker to help you. The most important thing is to try to enjoy it - choose films, tapes etc that you can enjoy, and if you get a chance to practise conversation, enjoy the conversation and don't worry about correct grammar (yet.) Good luck!