Nothing will improve your vocabulary and grammar skills as much as reading books will. And it's even better if you use an ebook reader because you can look up definitions much much quicker than a regular dictionary.
Enid Blyton and Roald Dahl are excellent fun authors to start with.
Add in a website like dict.cc or Forvo where you can check correct pronounciation of words and definitions, and you'll be off to a very good start. The perfect complements to this are watching TV shows and speaking the language yourself.
If you're wanting to learn more about English in an English interface then I'd recommend trying out https://www.noredink.com/. It is great for learning and practicing grammar and you can do quite a bit with their free accounts.
Khan Academy's grammar section (https://www.khanacademy.org/humanities/grammar) might also be worth checking out.