There are large amounts of dictionaries on the internet. Many of the best ones are free and offer very useful service. Online dictionaries from famous publishers such as Cambridge University Press are usually very accurate.
Beware of some free dictionaries that allow users to type their own definitions of words, they may not always be correct.
The Oxford Dictionary is a thpical example of the best dictionaries in British English. For pronunciation the best recommended dictionary is "English Pronouncing Dictionary by Daniel Jones".