This site takes you on a whirlwind tour of English pronunciation. Most of it refers to Received Pronunciation (RP), which is considered "standard" in the UK, while other resources refer to Standard American English. Our standard in Singapore is Standard Singapore English (SSE) but since reference materials for that is scarce, we'll take RP as our main guide here. You may find it useful to contrast RP with Colloquial Singapore English (CSE aka Singlish).
This site has three main sections, besides this page. The Phonemic Chart introduces the phonemic symbols used in RP and provides you with some Flash charts you can click on to listen. The Segmentals section briefly explains some of the features of English pronunciation at the level of phonemes, i.e. the basic sounds that make up the language. Under Suprasegmentals, you will find out more about English pronunciation features at a phrase or sentence level.
Below are some other resources you can use.
To check the pronunciation of a word, try these online dictionaries.
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A good way to practise pronunciation is to listen to yourself. Try it out now by recording yourself on your mobile phone.