Okay. First go into your cPanel and the filemanager. Go to your public_html folder, open it, and then delete everything inside of it. Don't delete the public_html folder itself because it's an important folder that shows your site online.
After that, go to PHPMyAdmin and delete all existing databases. Then go to MySql and delete any existing users.
When you complete this, then your account is "cleared" and ready for you to start again.