In order to upload the index page to your public_html folder you should perform the following actions:

1. Open your public_html folder, find index.html file ( or index.htm, index.php )- all you need is to point out the name of the start page of your site ( e.g. mainpage.html);

2. Enter your cPanel and click the icon "File manager" with the help of which you can upload this index page to your public_html folder;

3. After uploading this file via FTP or File manager you need to check if everything was done correctly entering in the browser the following link: http://yourdomain/index.html  where index.html is the main page of your site;

4. Then you need to access your .htaccess file (create it in you public_html if you haven't one) and write the following string right here: DirectoryIndex mainpage.html. 

Note: If your main page has filename like index.php, index.html or index.htm - you do not need to change DirectoryIndex statement.