Hi, Yuto. If you are looking to improve your conversational English, I would definitely get a language partner, with whom you can practice speaking English regularly. If you are looking to improve other aspects of the English language, I recommend enrolling in a reputable English class or getting an English textbook. I'm new to italki so I don't know much about its features, but I did see a professional teacher tab so maybe you can make use of those if you are looking for more formal training. Best of luck to you!