Create a .htaccess file, include the below information into it and upload it to the public_html directory. If you cannot create a .htaccess file because you are using Windows, simply name it htaccess and rename the file after it is uploaded with your ftp client.
This would cause filename.html to be treated as your default page, or default directory page. You can also append other filenames to it. You may want to have certain directories use a script as a default page, that is not a problem either.
DirectoryIndex filename.html index.cgi index.pl default.htm
In some cases the default file does not exist and you will wanto to have the default page be a URL or address. In such a case use the following:
RewriteRule ^/?$ /home [L,R=301]
/home being the URL you would like to set as the default address for the domain.