Use a dictionary as a basic tool for building English vocabulary. Keep a dictionary handy while you read books or newspaper articles. Look up words you don't know immediately so that you don't forget to do it later. Observe how words are used in context. Use an online dictionary if you don't have a print version.
Use a thesaurus to find synonyms and antonyms for common words. This will help expand your vocabulary by forcing you to seek alternative words. Use a thesaurus when you work on crossword puzzles.
Use flash cards to learn a large number of new words in a short period of time. Tests such as the SAT and GRE require a large vocabulary, and study materials may include lists of words likely to appear on tests. Create flash cards of these words as a help when drilling for a test.
Read to build your vocabulary. Expose yourself to large volumes of words, some of which will be unfamiliar. Learn some new words subconsciously simply by reading them in context, but take the time to look up new words.
Play word games to help improve your vocabulary. Games like crosswords puzzles, Boggle, Scrabble and Bananagrams all involve using vocabulary. Play with others to expose yourself to new words they use. Motivate yourself to learn new words to improve your skill at word games.