You might want to have a look at a thesaurus. They are arranged in categories of synonyms and antonyms. They can be found on-line, but the best way to really see how they are arranged is by looking at one at your school or public library. Roget's Thesaurus is the absolute best, and a great way to expand your vocabulary.
Are you looking for synonyms and antonyms? Synonyms are words that have similar meaning. For example happy and joyful are synonyms. Antonyms are words with different or opposite meanings. For example, happy and sad. Knowing synonyms and antonyms can help you expand your vocabulary, and avoid overusing certain words.