Your English looks rather good to me. There is an abundance of interactive websites available for free; you've found this one, so you're at least aware of their existence. Google really is the instrument of choice for locating others. And as for otherwise improving your English, I'd suggest reading as many books by Charles Dickens as possible, starting with Oliver Twist. You might also enjoy reading My Family and Other Animals, by Gerald Durrell.
Student or Learner