Your server software cannot find your default home page file in
your document root directory.
Most servers on web hosting typically run Apache on a Linux system.
If this IS the case and you will most likely have a
folder named 'public_html' or 'www', possibly both.
In most cases www is simply a symbolic link to public_html, so
either will serve the same purpose, this is your document root.
The root of your website basically, not to be confused with the
root of the filesystem, that is typically a couple of levels down.
The web folder in some cases may be at '/var/www', this most likely
the case if you have root access, which you will most certainly not
on shared hosting. Dedicated hosting, or your own machine, yes.
Place your index.html or index.php in your document root as stated
above.
Simply entering your domain name should now serve your homepage,
index.html/index.php.
Well done!