It depends. Are you looking for just a general, unspecific list of synonyms, or do you have specific words in mind. Words that may seem dull and you want to make your writing more colorful? If you have a specific word in mind that you would like to ‘brighten’ then a thesaurus would be the best place to look. You can also find synonyms in the dictionary, usually after the definition. In my opinion, it is the best to start with specific words in mind, otherwise a lot of time will be wasted looking for words that may not fit the bill. Good luck!