Your Instagram username is shown at the top of your Instagram profile and appears in your personalized Instagram URL, e.g., www.instagram.com/yourusername. It must be less than 30 characters and can only be made up of letters, numbers, periods, and underscores

You can find more about how to find your Instagram username in the: https://m.facebook.com/help/instagram/369207764224422

Go to your profile tab and click the gear icon on the top of the page. Go to "Privacy and Security" and uncheck the "Private Account" box.
If you are still struggling please see this video on YouTube: https://m.youtube.com/watch?v=j0XzeaROZq8