Changing a file's location

Hello, I'm very new to using ftp and publishing/building websites. I have to change the location of a file on my server to make it my home/index page. The page currently displayed when you type www. mysite .com is not the correct page that should be displayed. How can I change the location of a file to make it display when you type my website's URL into a browser?

I've tried finding an answer for this by googling the problem in so many different wasy but can't seem to find the correct information. And I'm fairly sure it's a simple answer, well I hope

Thank you for your help