Synonyms and antonyms

Synonyms are words with similar meanings and antonyms are opposites. An easy way to remember is that synonyms and similar both begin with s. You can buy dictionaries of them (e.g. Penguin) and this is a good way of improving your vocabulary but take care. Even words with close approximations differ as to register and function and so it is best to encounter them when reading before using them yourself. There is also the famous Roget's Thesaurus which gives phrases as well and is a fascinating book in its own right but even more liable to lead to errors of style.