No offense if this is anyone's method of expanding their vocabulary, but please, please,
please nobody go to a thesaurus/dictionary because you don't think you know enough words. A thesaurus is fine for verifying that two words mean close to the same thing, but the key to using new vocabulary effectively in writing is to have a good feel for the
connotation and
tone of a word, which a thesaurus will not help you with. You'll end up using, say... (I'm going to put this in spoiler tags because it's kind of off-color, but it's a good example of how dangerous using a thesaurus can be. This is from Merriam-Webster.)
Instead of going to a thesaurus/dictionary, I would suggest simply reading more books, whether fiction or non-fiction. Heck, this is one case where I'd say watching TV and movies or listening to the radio can help (depending on the sophistication of the material), especially if you're not a native English speaker. Go ahead and make a note of new words you encounter and look them up if you don't know the definition, but it's just as important to pay attention to how the word is used in context. Is it colloquial, sophisticated, crude, hip, archaic, or something else entirely?
tl;dr: Just broaden your general exposure to the language and your vocabulary will take care of itself over time.