Newspapers, news shows, discussions, documentaries - like the BBC, CNN, Discovery etc. Radio. Movies and shows are good but too much colloquialism or slang could be a problem. I mean that's how some of us in India get it right, although some of us have studied English literature throughout school. I don't think grammar books or "How to speak" books ever help. You've got to read, read and read and then converse. And fear of public speaking - well that's a different matter
I think you could help yourself by immersing yourself in such content. Just observe closely as to how they speak, enunciate, pay attention to the diction though "accent" per se, is not the issue. Your writing looks quite good, so I think it will not be difficult for you. Assuming you begin in September this year, you've got plenty of time. Good luck