How to exclude robots (google,yahoo) from my site

Put a file in the root of your website folder.
Put this in the file if you dont want your site indexed.

User-agent: *
Disallow: /

For more info about robots:
http://www.robotstxt.org/wc/robots.html
And for syntax:
http://www.sxw.org.uk/computing/robots/check.html