An English dictionary helps your pronunciation in two main ways
1. It shows you the phonetic pronunciation of every word
This means you can find out how to say the word - even if you don't have access to either a native speaker, or the audio of the word.
To find out how to say the word, look at the phonetic transcription of the word (which you can see right next to the word itself in the dictionary).
So for example, next to the word 'bed' you will see the phonetic transcription /bed/.
Go to the beginning of the dictionary (in the Longman Dictionary of Contemporary English - the one I use - it's inside the jacket cover) to see the pronunciation table.
Here you can see the pronunciation of every sound. So /b/ is the sound as in "back", /e/ is the sound as in "bed",and /d/ is the sound as in "day".
The pronunciation table shows you consonants and vowels. The vowels are divided into short, long, and dipthongs (which are two vowel sounds together).
This means that when you find a new word, you can work out how to say it, sound by sound.
2. It shows you the word stress
Word stress is a vital element of English pronunciation. In words with more than one syllable, you need to know which syllable has the main stress - and which syllables are not stressed. If you get the stress wrong (and you stress the wrong syllable, for example) the other person might not understand you.
You can check the stress of a word by looking for the apostrophe mark '. In the phonetic transcription of the word, the syllable that comes after ' is the stressed syllable.
'veg - ta - ble (the first syllable "veg" is stressed)
ve - ge - 'ta - ri - an (the third syllable "ta" is stressed)