The big majority of students who make well in the Cambridge Proficiency Examination have learnt to use a Good monolingual dictionary effectively. Such Dictionaries provide informations, not just about the Missings for the words but about their pronunciation and grammar as well. A student who studies how to use a dictionary  effectively will be able to work independently for much of the time, and will gain considerable insight to the workings of the English language. He or she will be able to confirm to the meanings for the words in a text where contextual clues are insufficient, pronounce words accurately by studying the phonological transcriptions, And use words accurately both when speaking and Writing. Make sure you make the room for at least one Good monolingual dictionary on your bookshelf- and then make sure you use it at a regular basis!