http://www.google.com/search?q=prevent+google+from+indexing+my+site

The same techniques will work for the major search engines.

Thank you...
Do I need to put it only on the main page (index.php) or in every page on the site (it's almost impossible)

Yoram

just use notepad and create a text file name it robots.txt with these two lines and put it in your root of your domain.

This disallows google

User-agent: google
Disallow: /

This disallows all

User-agent: *
Disallow: /

This allows all

User-agent: *
Disallow:

Just that simple...

@Yoram : You can add this to the header.php file. So it will be applicable to all pages.