AskOxford.com reports that there are at least a quarter million words in the English language.
The average English speaker only utilizes a small fraction of words but as writers we tend to have an insatiable lust for a larger lexicon.
Step 1: Read
Rule numero uno is and always has been, read. A writer must be a reader, a voracious, spellbound bookworm. That is, after all, where all the good words are! Read different genres and disciplines for each has its own unique vocabulary. Read novels, histories and poems. Read nonfiction, magazines and blogs. Go word hunting through as many literary landscapes as possible.