Use a good English dictionary

You need a good English dictionary. An English dictionary is the most important thing that you will need when learning English. Successful English learners use their dictionaries all the time — that's how they learn to use new words.
How to buy a good English dictionary?
1. It has to be an English-English dictionary.
2. It must give phonetic transcriptions (pronunciations) for every word.
3. It must give example sentences for every word.
4. It should be a software dictionary.
Example sentences: more important than definitions: If you want to improve your speaking/writing ability, read the example sentences in your dictionary. They show you how to use a word and they program your brain with correct English.

Dictionary reviews

Comparative review of dictionaries for English learners — Cambridge Advanced Learner's Dictionary, Collins COBUILD Advanced Dictionary (5th and 6th edition), Longman Dictionary of Contemporary English, and Oxford Advanced Learner's Dictionary. Which one is best?
Comparative review of English pronunciation dictionaries — Tom compares the Cambridge English Pronouncing Dictionary and the Longman Pronunciation Dictionary.
Collins COBUILD Advanced Learner's English Dictionary (4th/5th/6th edition) with CD-ROM
Random House Webster's Unabridged Dictionary CD-ROM — a cheap, very comprehensive dictionary of American English with phonetic transcriptions and American recordings.
Cambridge English Pronouncing Dictionary (17th edition) with CD-ROM — useful when you have doubts about a transcription in your normal dictionary.