There are many things that you can do to improve your English. Let me suggest three of them.
First, you should be an avid reader and try to read broadly. As you probably know English literature is broad in every sense of the world. For example, there are the tragedies of Shakespeare and the political theory of modern authors and even the modern playwrights such as Arthur Miller.
Second, since you want to be a teacher, you should also take some courses on education as it focuses on teaching English. Your university should offer courses.
Finally, you should seek an internship at a school. Many schools offer programs such as this. In this way, you can be a student teacher.