I don't like others simply realize that the site which I have created is made by Drupal or other CMSs. I know it's inevitable by some plugins like Wappalyzer in Firefox comming into play. But now I need to make it a bit harder so that at least amateurs can not realize it.
now if you right click on the site and select view page info in Firefox it will say that it is a Drupal site.!
how can I make it harder to realize?
Sounds like you are you talking about the generator meta tag. This is added by the metatag module by default. Go into the settings for metatags (admin/config/search/metatags) and set the generator tag to an empty string.
But if you really want to stop people knowing its a drupal site, it's going to be much much harder. Things off the top of my head that would indicate a drupal site are:
CHANGELOG.txt in the root of the site.
X-Drupal-Cache header.
Expires:Sun, 19 Nov 1978 05:00:00 GMT header.
Path to theme css and js files.
Actually, the path to any file on the site.
So I wouldn't even bother trying to hide the fact that its Drupal.
To remove the generator meta tag without the metatag module, add this to your theme template.php file (changing YOURTHEME to the name of your theme).
function YOURTHEME_html_head_alter(&$head_elements) {
unset($head_elements['system_meta_generator']);
}