“Words—so innocent and powerless as they are, as standing in a dictionary, how potent for good and evil they become in the hands of one who knows how to combine them.”—Nathaniel Hawthorne
I’d recommend Dictionary.com or Merriam Webster for their comprehensive coverage of language. They offer not only definitions & parts of speech, but also example usage, related forms, synonyms, idioms, origin, and history. Grab their apps (Dictionary: Android, iPhone; M-W: Android, iPhone) and whenever you see a new word, look it up, write it down (if possible), and use it. You may also enjoy the Oxford Learner’s Dictionary.
Whenever I learn a new word, I look up its etymology on Wiktionary or Etymonline (whose architect I respect tremendously.) This will help you understand definition more precisely. Pay special attention to roots & affixes (i.e. prefixes and suffixes). Here are two more root lists from Michigan State and MyVocabulary. Studying word components (roots & affixes) will quickly grow your vocabulary.