If you see the "Index of /" page and a list of files that you have uploaded it means that the first page of your site isn't named index.htm, index.html or index.php. The server, therefore, doesn't know which page you would like as your first page.
Note that the server is case sensitive so you may have a page called Index.html or INDEX.HTML which will not work. The file name will need to be all lowercase.
If you see the "Index of /" page and a folder containing your site it means that you have uploaded your site one layer too deep.
Incorrect file structure: public_html/folder containing your site/
Instead of uploading a folder containing your site, you will want to open that folder and upload the contents directly to the public_html folder.
Correct file structure: public_html/
If you see the "Index of /" page and none of the files you have uploaded login to your file manager and make sure that you've uploaded your pages into the public_html directory. Chances are that you've uploaded your files outside of your "public_html" directory and they aren't visible to the public. To fix that, reload your pages inside the public_html directory.