Open a word processing program and right-click anywhere on the word. Software such as the Microsoft Word program, part of the Microsoft Office Suite, offers a right-click option called "Synonyms." Review the bottom of the fly-out list for the word "antonym" in parenthesis next to the word's antonym. Note that the program's antonym listing may be limited and will not have antonyms for proper nouns, places and slang.
Look up the word in a text such as the "Scholastic Dictionary of Synonyms, Antonyms and Homonyms," "The Merriam-Webster Dictionary of Synonyms and Antonyms" or "The Oxford Dictionary of Synonyms and Antonyms." Many regular dictionaries also include antonyms; search for the word in alphabetical order and check the last part of the listing.
Type the word into a free online antonym website such as Synonym, which has a separate section for antonyms, both alphabetical and by text search, and Thesaurus, which lists antonyms at the end of the synonyms. Synonym offers 19 options of antonyms for the word "soft," for example, with over 25 options showing up on the Thesaurus website.