Here are some lists of the most common words in English. You can go through them to see how many you know and learn the ones you don't.
http://www.learninggamesforkids.com/vocabulary_games.html
Online interactive vocabulary and spelling games to expand your vocabulary and improve your vocabulary skills.
http://www.englishpage.com/vocabulary/vocabulary.html
Learn some advanced vocabulary and practice using them in context. There are also exercises to help you use the new words with other words that you know to make them easier to remember.
http://dictionary.cambridge.org/dictionary/learner-english/
Learner dictionaries are designed for English learners. The definitions are in easier English and there are examples to show you how to use the new word in context. You can also listen to the pronunciation of the words to learn how to read them aloud.
https://en.oxforddictionaries.com/english-thesaurus
You can find synonyms (words with similar meaning) and antonyms (words with opposite meaning) of a word in a thesaurus. Use it to make your writing more interesting.